Minor tweaks to UserspaceRebootLogger

* Add method name to all Slog.wtf() calls
* Do not Slog.wtf in shouldLogUserspaceRebootEvent method
* Also added OWNERS for UserspaceRebootLogger.java

Test: device boots
Test: no UserspaceRebootLogger logs in logcat
Test: atest CtsUserspaceRebootHostSideTestCases
Change-Id: Ifc2a13957157838dade3701f0b35dad51d26ec45