Log watchdog-initiated rollback reason

To help with monitoring Mainline releases, log the reason
for a watchdog-initiated rollback. This may be due to
native crashes, app crashes, ANRs or explicit health check
failures.

Add a mapping from PackageWatchdog failure reason to the
new metrics.

Bug: 138782888
Test: atest PackageWatchdogTest
Test: atest StatsdHostTestCases
Change-Id: Ia3e73d955508297004591eac762555665c557b8a