Fix latching of final buffer when moving off-screen and destroying

Previously processing of mLayersPendingRemoval was happening after
doTransaction, but now it is happening before. doTransaction is
still responsible for latching mDrawingStateModified =
mDrawingState.modified. The distinction between the two is now rather
meaningless and should be eliminated in a future CL, for now we
simply unblock the check, by ensuring hasFrameUpdate will return
true even if we haven't called doTransaction().

Test: Existing tests pass. CtsWidgetTestCases.
Bug: 191514384
Change-Id: I6a18452020a3098a6c16b7c0d36a3c0f2b93a8fd