Add immutability flag to PendingIntents

Explicitly set FLAG_IMMUTABLE for all PendingIntents
in BugreportProgressService.

Test: Builds
Bug: 170165227
Bug: 171830604
Bug: 178803845
Change-Id: Iae1489b9c8fa323448e60615e99ad96aee1cd1d7
Merged-in: Iae1489b9c8fa323448e60615e99ad96aee1cd1d7
(cherry picked from commit 2d38f6e4)