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/java/com/keylesspalace/tusky/settings/ProxyConfiguration.kt

@@ -22,7 +22,7 @@ class ProxyConfiguration private constructor(
         }
         fun isValidHostname(hostname: String): Boolean =
             IP_ADDRESS_REGEX.matches(hostname) || HOSTNAME_REGEX.matches(hostname)
-        const val MIN_PROXY_PORT = 0
+        const val MIN_PROXY_PORT = 1
         const val MAX_PROXY_PORT = 65535
     }
 }