Grab audio focus when listening to voicemail.

Music and other audio should be paused when listening to a voicemail.
The voicemail player grabs transient focus so that when the voicemail is
paused or finished, the focus returns to the music that was playing.

Bug: 19589110
Change-Id: I9876cc2700139d0db787d0e370020d2a38d82cda