Clear the calling identity in isUsableDisplay.

The INTERNAL_SYSTEM_WINDOW permission is checked when
WindowManagerService#shouldShowSystemDecors is called, we should leave
this permission check because it is public method.
Clear the calling identity in WallpaperManagerService#isUsableDisplay
as this method should be used only internally.

Fix: 136447676
Test: atest VrDisplayTests MultiDisplaySystemDecorationTests
Change-Id: I97a93cdd0253933527010f97049694b59d5a4e2a