Define FLAG_MUTABLE and add a compat change for explicit definition of...

Define FLAG_MUTABLE and add a compat change for explicit definition of PendingIntent mutability flags

For future releases, we will require that either FLAG_IMMUTABLE or
FLAG_MUTABLE is defined when a PendingIntent is created.

The check only results in a log.wtf for now while we notify, identify,
and update major PendingIntents. We will add a stricter
IllegalArgumentException soon.

- Define FLAG_MUTABLE
- Add a check and log.wtf that either FLAG_IMMUTABLE or FLAG_MUTABLE is
  specified on creation for future releases
- Update api/current.txt and non-updatable-api/current.txt to include
  FLAG_MUTABLE

Test: atest PendingIntentTest
Bug: 160794467
Change-Id: I1f1f72c6ce22358e98e9f38be2fb98b27e60898c