Verify pending intent flags

All PendingIntent included activity and broadcast should set
FLAG_IMMUTABLE flag.

Bug: 156353008
Test: atest TetheringTests

Original change: https://android-review.googlesource.com/c/platform/frameworks/base/+/1325696

Change-Id: I885046eb25c6dafbcb4284fd94bff0052f7bdd09
Merged-In: I4592e914b67ecb8865d7de47797423f27e9ff840
(cherry picked from commit 1a306394)