(this one is for @charlag)
Calling `PreferenceManager.getDefaultSharedPreferences()` will read the
preference file from disk every time. This PR makes `SharedPreferences`
a singleton so they will only be created once at appstart (with a few
exceptions where it is hard to inject, e.g. in the `openLink` helper)
which should help getting our ANRs down.
```
StrictMode policy violation; ~duration=285 ms: android.os.strictmode.DiskReadViolation
at android.os.StrictMode$AndroidBlockGuardPolicy.onReadFromDisk(StrictMode.java:1666)
at libcore.io.BlockGuardOs.access(BlockGuardOs.java:74)
at libcore.io.ForwardingOs.access(ForwardingOs.java:128)
at android.app.ActivityThread$AndroidOs.access(ActivityThread.java:8054)
at java.io.UnixFileSystem.checkAccess(UnixFileSystem.java:313)
at java.io.File.exists(File.java:813)
at android.app.ContextImpl.ensurePrivateDirExists(ContextImpl.java:790)
at android.app.ContextImpl.ensurePrivateDirExists(ContextImpl.java:781)
at android.app.ContextImpl.getPreferencesDir(ContextImpl.java:737)
at android.app.ContextImpl.getSharedPreferencesPath(ContextImpl.java:962)
at android.app.ContextImpl.getSharedPreferences(ContextImpl.java:583)
at android.content.ContextWrapper.getSharedPreferences(ContextWrapper.java:221)
at android.content.ContextWrapper.getSharedPreferences(ContextWrapper.java:221)
at androidx.preference.PreferenceManager.getDefaultSharedPreferences(PreferenceManager.java:119)
at com.keylesspalace.tusky.BaseActivity.onCreate(BaseActivity.java:96)
...
```
d554d71958