Keep FakeFeatureFlagsImpl

* Keep FakeFeatureFlagsImpl.
* Use EnableFlags and DisableFlags annotation
* Switch back from `assumeTrue` to `assert` when flag is enabled or disabled.
* Remove flag rollback test.

Bug: 342830919
Change-Id: Ic9b3438f3f0e31e64d5405ed81903b352195714a

proguard.flags

@@ -65,3 +65,6 @@
 -keep class androidx.window.extensions.** { *; }
 -dontwarn androidx.window.extensions.**
 -keep class androidx.window.** { *; }
+
+# Keep the com.android.settings.media_drm.FakeFeatureFlagsImpl
+-keep class com.android.settings.media_drm.FakeFeatureFlagsImpl { *; }