PackageManager: Restrict system packages to protected storage

Don't allow packages signed with the platform key to be
installed into user (or app) controlled storage

Change-Id: I7390b3bdf5289411f7a335c69635bed9f74bbfb3
Prevent audio players starting on startup.

Previously, (at least) Apollo was started on startup (in that it
appeared in the notification bar) because it listens to the AUDIO_BECOMING_NOISY
intent to silence itself in that case. This intent is broadcast by
AudioService when there's a user switch to silence any potential audio
players for the old user. As the system also sends around a
USER_SWITCHED intent on startup, this led to the aforementioned player
startup.
As the intent of AudioService was to act on a user backgrounding anyway,
send out AUDIO_BECOMING_NOISY as reaction to a USER_BACKGROUND intent
instead of the USER_SWITCHED intent.

Change-Id: Iba35844a1274c9dd195f104068304e6a7ffa809e