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/model/Tunnel.java

@@ -49,7 +49,8 @@ public class Tunnel extends BaseObservable implements Keyed<String> {
         return manager.delete(this);
     }
 
-    @Bindable @Nullable
+    @Bindable
+    @Nullable
     public Config getConfig() {
         if (config == null)
             manager.getTunnelConfig(this).whenComplete(ExceptionLoggers.E);
@@ -81,7 +82,8 @@ public class Tunnel extends BaseObservable implements Keyed<String> {
         return TunnelManager.getTunnelState(this);
     }
 
-    @Bindable @Nullable
+    @Bindable
+    @Nullable
     public Statistics getStatistics() {
         // FIXME: Check age of statistics.
         if (statistics == null)