Trust System UI owned display to receive keys

A11y service cannot get focus of bubbles because it's not a
System owned display. This patch makes System UI owned display
a trusted display. Moreover, this patch refactors the logic to
identify a trusted display by introducing FLAG_TRUSTED and
removes the trusted display check along with supportsSystemDecorations()
because the check has been included in supportsSystemDecorations().

fixes: 155823002
Bug: 152416787
Test: atest DisplayContentTests
Test: atest WindowFocusTests
Test: atest TaskDisplayAreaTests
Test: atest MultiDisplaySystemDecorationTests
Test: atest DisplayTest
Change-Id: Ie684c6488904e5aa8cae166a455c6d55455e5f55