Dump recent shouldListenForFace() calls in bugreport

Keeps track of recent successful calls to shouldListenForFace() in
KeyguardUpdateMonitor and dumps the relevant data into a bugreport file
when requested by the system. This will allow us to better debug the
issue reported in b/157630051.

Test: Set `DEBUG_SPEW = true`, look for log output in logcat
Test: Run `adb bugreport`, look for log output in bug report

Bug: 157630051
Change-Id: I5021534069d59158dde4488ebb13ef90d339ee39