Clean up flag notification_async_hybrid_view_inflation 1

The impact of this flag is particularly wide, so breaking down the flag
removal by method for code robustness and easier review. This change
cleans the isUnexpectedlyInLegacyMode() method of flag-helper class.

Clean up flag:
com.android.systemui.notification_async_hybrid_view_inflation

Bug: 409612328
Test: SingleLineViewInflaterTest, SingleLineViewBinderTest,
NotifUiAdjustmentProviderTest
Flag: EXEMPT flag cleanup

Change-Id: I7f9ee62d63882e2767830bf4a4e5d2b30185f95d