Add FLAG_IMMUTABLE to PendingIntents

If your app targets Android 12, you must specify the mutability of each
PendingIntent object that your app creates. This additional requirement
improves your app's security.
https://developer.android.com/about/versions/12/behavior-changes-12#pending-intent-mutability

Android Lint is being updated, and adds an error-level check for this.

Bug: 247885568
Test: Presubmits
Change-Id: Ib82e2b0bbea3c1dd1810c56c4ea70f7809aa122a