[FM] FM Radio should reduce volume when getting AudioManager.AUDIOFOCUS_LOSS_TRANSIENT_CAN_DUCK

When using Talkback (a11y app for visually impaired end users)
with the FM Radio app, it can be hard to hear the Talkback audio
feedback as the FM Radio output is mixed in with Talkback.

The fix is to handle this issue with AudioManager.AUDIOFOCUS_LOSS_TRANSIENT_CAN_DUCK.

Bug 16737172

Change-Id: I836c6a298918829645f153768c0f0fa6afc5429c
Signed-off-by: Benson Huang <benson.huang@mediatek.com>