Updated onSharedPreferenceChanged listener behavior.

The onSharedPreferenceChanged listener will now also be called on
Editor#clear with a null key.
This change is gated behind a flag using the new app compat framework.

Removed the OnSharedPreferencesClear listener interface.

Bug: 119147584
Bug: 138293946
Test: atest android.content.cts.SharedPreferencesTest
Change-Id: Ieea168eb40afb8f1b5830f1541be20d93d6f94b5