Implement HTTP proxy summary as a SummaryProvider (#3091)

* Handle preference fragments using the framework

The previous code started new preference "screens" as activities, even though
each one hosted a single fragment.

Modify this to use the framework's support for swapping in/out different
preference fragments.

PreferencesActivity:
- Remove the code for launching tab and proxy preferences
- Remove the code for setting titles, each fragment is responsible for that
- Implement OnPreferenceStartFragmentCallback to swap fragments in/out with
  the correct animation

PreferencesFragment:
- Use `fragment` property instead of `setOnPreferenceClickListener`
- Set the activity title when resuming

Everything else:
- Set the activity title when resuming

* Implement HTTP proxy summary as a SummaryProvider

Uses the frameworks's support for setting summaries instead of rolling our
own.

Also fix a tiny bug -- the minimum port number to connect to should be 1,
not 0.

* Lint

app/src/main/res/values/strings.xml

@@ -291,6 +291,9 @@
     <string name="pref_title_http_proxy_server">HTTP proxy server</string>
     <string name="pref_title_http_proxy_port">HTTP proxy port</string>
     <string name="pref_title_http_proxy_port_message">Port should be between %d and %d</string>
+    <string name="pref_summary_http_proxy_disabled">Disabled</string>
+    <string name="pref_summary_http_proxy_missing">&lt;not set></string>
+    <string name="pref_summary_http_proxy_invalid">&lt;invalid></string>
 
     <string name="pref_default_post_privacy">Default post privacy</string>
     <string name="pref_default_post_language">Default posting language</string>