Make test compatible with relative insets flags

This change removes the original verification of the caption insets but
instead using a flag awared method to verify the behavior for both of
the flag values to make the test compatible with the flag.

Flag: com.android.window.flags.relative_insets
Bug: 420611475
Bug: 277292497
Test: WindowDecorationTests
Change-Id: I6abb9164a9c04f2b903cfffab14181f614ff21c0