Remove the obsolete KeyguardState.showingAndNotOccluded.

There used to be a isShowingAndNotOccluded() function which was added
when this field was added (ag/440769). This is no longer the case. Plus,
an occluded field was added in ag/15924835.

The field isn't set correctly for dumping. Causes confusion for
debugging.

Bug: 15924835
Bug: 232985643
Test: no-op cleanup, passes existing tests; DumpSys.
Change-Id: Icf16a48f70d40ec9be09c3bc6ec987249f75d332