Introduce getWindowContextToken

Also consolidate UI context related flags to ContextType.

Bug: 171280916
Test: atest ContextTest ContextAccessTest
Test: atest WindowContextTest WindowContextTests \
            WindowMetricsTest WindowMetricsTests
Test atest InputMethodServiceStrictModeTest \
           InputMethodMenuControllerTest
Test atest InputMethodServiceTest#testGetDisplay
           StrictModeTest#testIncorrectContextuse_*

Change-Id: I7f3a7f951c664d4c329059942e688ee3a625b992