Disable 3-button-nav buttons during back button hold

Bug: 373544911
Test: TaskbarNavButtonControllerTest
Test: Manual, i.e. verify that pressing buttons in 3-button-nav while the back button is pressed does not have any effect
Flag: com.android.window.flags.predictive_back_three_button_nav
Change-Id: I5abad5f2f74d09c790380a2eeb27aff3b780b925
3 files changed