Add new handler mechanism in MountService to make unmount api asynchronous.

MountService updates state on PackageManager and then tries to
kill processes holding file references to media about to be unmounted by
invoking api on ACtivityManager. This is retried upto 4 times to make sure all
processes holding file references are killed before unmounting the media
at specified path.

Also changed PackageManger api to return boolean value to indicate if
MountService is likely to receive broadcasts related to apps on sd.