Mark implicit PendingIntents as immutable

Bug: 156020795
Test: manual, atest
Change-Id: I72206c7a52b067b77d6542d170a6483713dfeee7
(cherry picked from commit 84e08280d3882cfe4bad12ab426016c6d0efc7fb)
(cherry picked from commit 02bdeeffb3135de0f3e18c3a85e6730b021fd191)
2 files changed