Do not apply start transaction by letterbox transition observer

Otherwise the animation may be flickering because the transition
handler has not yet set the initial setup to start transaction.

The transition handler is responsible to apply the start transaction,
so remove the apply which is called from observer.

Bug: 438851781
Flag: com.android.window.flags.app_compat_refactoring
Test: Cold launch an app with
      LAYOUT_IN_DISPLAY_CUTOUT_MODE_SHORT_EDGES.
      Rotate the device to landscape.
      No flickering before the rotation animation starts.
Change-Id: I0b7d4b0404c898ce15bd5b62cc45010e1afb3c98