Turn PluginManager into a Singelton.

Fixes a memory leak around UncaughtExceptionHandler where
a new one would be created each time PluginManagerImpl was
instantiated.

Fixes: 199853969
Test: manual
Change-Id: Ie4c3a0ca791ded4bb9737bd24a554943990c6680

packages/SystemUI/src/com/android/systemui/plugins/PluginsModule.java

@@ -102,6 +102,7 @@ public abstract class PluginsModule {
     }
 
     @Provides
+    @Singleton
     static PluginManager providesPluginManager(
             Context context,
             PluginActionManager.Factory instanceManagerFactory,