Synthesize app PendingIntents rather than wrap them

Sometimes it's important that the system create PendingIntents that are
owned by their target app, rather than by the system itself.  Make that
possible, and use them in the Notification auto-grouping mechanism.
This makes the auto group Notification's undering PendingIntent how has
the same capabilities as the app whose notifications it elides, rather
than having the system's own capabilities.

Along the way, build the PendingIntent mutability-check exception string
only when it's going to be used, not every time any PendingIntent is
created.

Bug: 174243774
Test: atest CtsAppTestCases:PendingIntentTest
Test: atest CtsAppTestCases:NotificationManagerTest
Test: atest NotificationManagerServiceTest
Change-Id: I5ec03706bc439d305b0093a1ee9c5d381024029c