RebootEscrow: create event log

When an event happens, log it to the event log for later
dumping in a bugreport. This gets around the issue where
events were scrolling off the logcat before they became
relevant for debugging the issue.

Test: adb shell dumpsys lock_settings
Bug: 151512506
Change-Id: I0877f915479a52746ed7aeb72888deffeec81a69