NotificationPlayer: use AUDIOFOCUS_GAIN_TRANSIENT instead of AUDIOFOCUS_GAIN in looping mode

A voip app will usually use notification with FLAG_INSISTENT when a new voip call comes, if the duration hint was AUDIOFOCUS_GAIN the music app will not continue after the voip call end.

Signed-off-by: wumin3 <wumin3@xiaomi.com>
Change-Id: I6270bb08181b8fb94c84ec0f69bd226f3a7d0f01