Expose isUserKeyUnlocked as a TestAPI

This is needed to replace usage of the adb shell
command 'dumpsys mount' in some cts tests -to identify
if user key is locked- with a more reliable TestAPI

Bug: 173695151
Bug: 180328483
Test: N/A
Change-Id: Iebcc0080195b207f133e8d7186a098f39e137f42