Volume keys control music volume after music stops

On phones, add a short delay during which volume keys still
control music volume after music has stopped. This is similar
to the tablet behavior for notifications volume.
It allows users to control the prompt volume for talk back,
voice search or navigation even after the prompt has stopped.

Bug 7162364.

Change-Id: Iffbca9ec30ebcbe5846f783ddd260b6e736caf2b