Teach storage appops.

For modern apps targeting M SDK and up the external storage state
is deterined by granted permissions. For apps targeting older SDK
the storage access is determined by app ops correspning to the
storage permissions as the latter are always granted.

When app ops change we do not remount as we kill the app process
in both cases enabling and disabling an app op since legacy code
is not prepared for dynamic behavior where an operation that failed
may next succeed. Hence, we remount when we start the app.

For modern apps we don't kill the app process on a permission
grant, therefore we synchronously remount the app storage.

bug:22104923

Change-Id: I601c19c764a74c2d15bea6630d0f5fdc52bf6a5a