Only consider parent windowing mode if activity has no mode override

The freeform case could be
  task windowing mode=freeform
  activity requested windowing mode=undefined (inherit task)
Because isFloating returns false for undefined mode, it should ignore
the undefined case because the activity will inherit the mode from
task, then it should be considered as floating mode.

This is the follow-up of commit 79ea1505.

Bug: 151861875
Flag: com.android.window.flags.insets_decoupled_configuration
Test: adb shell am compat disable \
      INSETS_DECOUPLED_CONFIGURATION_ENFORCED test_app
      Launch test_app in freeform and resize it.
      The applySizeOverrideIfNeeded should not take effect.
Change-Id: I0153cbc5d53e1c05f41aa2423d271b0cded792b5