Apps on sdcard: Add new broadcasts

Add new broadcasts ACTION_MEDIA_RESOURCES_AVAILABLE and
ACTION_MEDIA_RESOURCES_UNAVAILABLE that get broadcast by
PackageManagerService when sdcard gets mounted/unmounted
by MountService so that packages on sdcard get recognized by
various system services as being installed/available or
removed/unavailable by the system.
The broadcasts are sent before the actual package cleanup which includes
mounting/unmounting the packages and we force a gc right after so
that any lingering file references to resources on sdcard get
released.