Add critical event log

The critical event log is a small (20 items) log of recent "critical
events" such as watchdogs and half-watchdogs.

Events from the log that occurred in the last 5 minutes are dumped to
ANR files and ANR dropbox entries in textproto format.

This change adds the log itself & support for logging watchdogs and
half-watchdogs. Other event types such as system server crashes will be
added in follow-up CLs.

Bug: 200263868
Test: atest CriticalEventLogTest
Change-Id: I431e15c5d5c16a358cd0eea30682078d04257499