Fix FAB reporting incorrect state

This CL also cleans up the flag
floating_menu_narrow_target_content_observer

Confirmed on my DUT that this CL fixes both of b/331740049 and b/376473165.

Bug: b/376473165
Test: atest MenuViewTest
Flag: com.android.systemui.floating_menu_notify_targets_changed_on_strict_diff
Change-Id: I73ff30a8920b681262f215c8426c26e6e4aa72d2