Fix exported broadcast receiver vulnerability

CellBroadcastReceiver was declared as exported in the manifest and
therefore allowed any app to send a MARK_AS_READ intent, even though
it's only supposed to be called from an internal PendingIntent. Fix this
by creating a new non-exported receiver and using that to handle the
mark-as-read intent instead.

Fixes: 162741784
Test: atest GoogleCellBroadcastReceiverUnitTests
Change-Id: I03c8163c22a6fbc92613ca2ccd2ac191fc0084a4
Merged-In: I03c8163c22a6fbc92613ca2ccd2ac191fc0084a4
(cherry picked from commit e514bc6a01fdd36a519fd4fefffa45f166911c97)
(cherry picked from commit db3208b69c1debcc0746df99a4c7cab02c3c52f0)
(cherry picked from commit e2a3f5f51bf6723d54599e5fa81577f6b9465116)
7 files changed