Perform preference schema upgrades at startup (#3186)

* Perform preference schema upgrades at startup

Over time it can be desirable to change how preferences are interpreted.

Preferences might be removed, or renamed. Or the default value for a
preference might be changed.

When this happens it's important that users upgrading from one version to
the next (or jumping from one version to several versions ahead) get a
consistent experience. In particular:

- Preferences that no longer exist should be deleted
- Preferences that have been renamed should have the old preference values
  copied over
- If the user used the default value for the preference, and the default has
  changed, the previous default value should be explicitly set as their
  value for the preference

To support this, store a SCHEMA_VERSION as a preference. This is not exposed
to the user, and corresponds to the app's VERSION_CODE.

If the version code does not match the schema version then this is a newer
version of the app with older preferences that may need to be changed.

Those changes will be implemented in `upgradeSharedPreferences`.

* Translated using Weblate (Hungarian)

Currently translated at 100.0% (552 of 552 strings)

Co-authored-by: Gera, Zoltan <gerazo@manioka.hu>
Translate-URL: https://weblate.tusky.app/projects/tusky/tusky/hu/
Translation: Tusky/Tusky

* Translated using Weblate (Chinese (Simplified))

Currently translated at 100.0% (552 of 552 strings)

Co-authored-by: Eric <alchemillatruth@purelymail.com>
Translate-URL: https://weblate.tusky.app/projects/tusky/tusky/zh_Hans/
Translation: Tusky/Tusky

* Translated using Weblate (Ukrainian)

Currently translated at 100.0% (552 of 552 strings)

Co-authored-by: Ihor Hordiichuk <igor_ck@outlook.com>
Translate-URL: https://weblate.tusky.app/projects/tusky/tusky/uk/
Translation: Tusky/Tusky

* Translated using Weblate (Vietnamese)

Currently translated at 100.0% (552 of 552 strings)

Co-authored-by: Hồ Nhất Duy <mastoduy@gmail.com>
Translate-URL: https://weblate.tusky.app/projects/tusky/tusky/vi/
Translation: Tusky/Tusky

* Translated using Weblate (Belarusian)

Currently translated at 100.0% (552 of 552 strings)

Co-authored-by: Mikalai <mikalai.hryb@gmail.com>
Translate-URL: https://weblate.tusky.app/projects/tusky/tusky/be/
Translation: Tusky/Tusky

* Translated using Weblate (Belarusian)

Currently translated at 100.0% (552 of 552 strings)

Co-authored-by: Andrej Zabavin <andre.zabavin@gmail.com>
Translate-URL: https://weblate.tusky.app/projects/tusky/tusky/be/
Translation: Tusky/Tusky

* Translated using Weblate (Belarusian)

Currently translated at 100.0% (552 of 552 strings)

Co-authored-by: Mikalai <mikalai.hryb@gmail.com>
Translate-URL: https://weblate.tusky.app/projects/tusky/tusky/be/
Translation: Tusky/Tusky

* Translated using Weblate (Belarusian)

Currently translated at 100.0% (552 of 552 strings)

Translated using Weblate (Belarusian)

Currently translated at 100.0% (552 of 552 strings)

Co-authored-by: xzFantom <xzfantom@gmail.com>
Translate-URL: https://weblate.tusky.app/projects/tusky/tusky/be/
Translation: Tusky/Tusky

* Translated using Weblate (Japanese)

Currently translated at 91.3% (504 of 552 strings)

Co-authored-by: TAKAHASHI Shuuji <shuuji3@gmail.com>
Translate-URL: https://weblate.tusky.app/projects/tusky/tusky/ja/
Translation: Tusky/Tusky

* Translated using Weblate (Icelandic)

Currently translated at 100.0% (552 of 552 strings)

Co-authored-by: Sveinn í Felli <sv1@fellsnet.is>
Translate-URL: https://weblate.tusky.app/projects/tusky/tusky/is/
Translation: Tusky/Tusky

* Translated using Weblate (Belarusian)

Currently translated at 100.0% (552 of 552 strings)

Co-authored-by: Mikalai <mikalai.hryb@gmail.com>
Translate-URL: https://weblate.tusky.app/projects/tusky/tusky/be/
Translation: Tusky/Tusky

* Lint

---------

Co-authored-by: Gera, Zoltan <gerazo@manioka.hu>
Co-authored-by: Eric <alchemillatruth@purelymail.com>
Co-authored-by: Ihor Hordiichuk <igor_ck@outlook.com>
Co-authored-by: Hồ Nhất Duy <mastoduy@gmail.com>
Co-authored-by: Mikalai <mikalai.hryb@gmail.com>
Co-authored-by: Andrej Zabavin <andre.zabavin@gmail.com>
Co-authored-by: xzFantom <xzfantom@gmail.com>
Co-authored-by: TAKAHASHI Shuuji <shuuji3@gmail.com>
Co-authored-by: Sveinn í Felli <sv1@fellsnet.is>

app/src/main/java/com/keylesspalace/tusky/settings/SettingsConstants.kt

@@ -16,6 +16,7 @@ object PrefKeys {
     // Note: not all of these keys are actually used as SharedPreferences keys but we must give
     // each preference a key for it to work.
 
+    const val SCHEMA_VERSION: String = "schema_version"
     const val APP_THEME = "appTheme"
     const val EMOJI = "selected_emoji_font"
     const val FAB_HIDE = "fabHide"