SystemUI: change back button to hide ime buttom with rotate animation.

Change-Id: Ia9a7716a6797d0442670451e59857d44b579b5f6
Allow ringer mode change to leave priority notification mode

AudioManager.RINGER_MODE_SILENT was re-mapped from to
Global.ZEN_MODE_IMPORTANT_INTERRUPTIONS, but this change:

Change: 806dbae9e12f3875cf0def3da2f3c3c2b768a844
Link: https://github.com/android/platform_frameworks_base/commit/806dbae9e12f3875cf0def3da2f3c3c2b768a844

This made it impossible to turn off zen mode by setting the
ringer mode to either AudioManager.RINGER_MODE_VIBRATE or
AudioManager.RINGER_MODE_NORMAL, as described in documentation:

https://developer.android.com/about/versions/android-5.0-changes.html

Issue: https://code.google.com/p/android-developer-preview/issues/detail?id=1780

This change restores that functionality.

PatchSet 2:
Simplify this to only check if zen mode is not already off

Signed-off-by: Bryan Emmanuel <piusvelte@gmail.com>