Player Component fixes

AI2 Player component has 2 issues raised by users:
1. Player allows playing tracks only in foreground by default, but some users prefer keeping tracks always playing even in background (as in AI1).
2. Player cannot pause automatically when there is an incoming phone call, or similar interruption. 

So here I proposed fixes on Player component:
1. A boolean property PlayInForeground to switch between the two modes (Foreground/Always), and it's false by default.
2. Player automatically detects and handles (pause for) external transient interruption that preempts the audio channel, such as phone calls, alarms.

Proposal details: https://docs.google.com/document/d/1-5E4fsaqw8SIPSJ5i7pGL0sCGNOoyMmUa132DMegN1o/edit?usp=sharing 

Open Source Post: https://groups.google.com/forum/#!topic/app-inventor-open-source-dev/F_B9dVex9WI