Remove StorageManager#isUserKeyUnlocked()

StorageManager#isUserKeyUnlocked() is no longer used, since all callers
have switched to using StorageManager#isCeStorageUnlocked() which has a
clearer name.  Therefore, remove StorageManager#isUserKeyUnlocked().

Note that isUserKeyUnlocked() is a @TestApi and isCeStorageUnlocked()
isn't.  This is fine because no CTS tests call either method, so
@TestApi is unnecessary.  And unlike regular APIs, Test APIs may be
added/changed/removed at any time.  Of course, we can always add
@TestApi to isCeStorageUnlocked() later if it actually becomes needed.

Bug: 306204742
Test: build
Change-Id: Ibf7dcccf85bdc566941d6c401ca8ad3d19fd6fc1