Remodel the Model

- The configuration and crypto model is now entirely independent
of Android classes other than Nullable and TextUtils.
- Model classes are immutable and use builders that enforce the
appropriate optional/required attributes.
- The Android config proxies (for Parcelable and databinding) are
moved to the Android side of the codebase, and are designed to be
safe for two-way databinding. This allows proper observability in
TunnelDetailFragment.
- Various robustness fixes and documentation updates to helper classes.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

app/src/main/java/com/wireguard/android/preference/VersionPreference.java

@@ -34,7 +34,8 @@ public class VersionPreference extends Preference {
         });
     }
 
-    @Override @Nullable
+    @Nullable
+    @Override
     public CharSequence getSummary() {
         return versionSummary;
     }