Don't call onModalnessUpdated if modalness stays the same

- Repeatedly calling onModalnessUpdated results in repeated calls of View.setAlpha, which caused regression in Overview loading time

Fix: 341266234
Test: android.platform.test.scenario.messenger.OpenAppFromQuickStepAndOpenQuickStepMicrobenchmark#testOpenQuickStepAndOpenApp
Flag: EXEMPT bugfix
Change-Id: Id756e80f20afafee2bb45e6a3ab861d83ab0b674