Audio continue to played even if paused manually

Currently, audio will resume playing after a phone call
or notification even if the user had manually paused playback.

This patch addresses that by introducing a new player state
to distinguish the user pausing playback and the browser
losing audio focus for some other reason. Audio will only
 resume if the browser temporarily lost audio focus and
not when the user has manually paused playback.

Change-Id: I9e8beaedb0fcc5afe920068297ed9c387eab2ac8
Signed-off-by: Roger Chen <cxr514033970@gmail.com>