Omit NavigationGuard if the input method uses FLAG_LAYOUT_IN_OVERSCAN

In some cases (e.g. Android Wear) SystemWindowInsets can be non-zero
due to overscan layout hints even when no SystemUI is present.

This change resepects the overscan flag on input method windows
allowing an IME to opt-out of the navigation bar guard and receive
full-height content view.

BUG: 32700226
Change-Id: Ic38f204a892bf34e8dae65990d5aa8c95af555d8