Limit letterboxing to CameraCompat in freeform.

DisplayRotationCompatPolicy should not change the
letterbox check as letterboxing is not a part of
that policy.

Flag: com.android.window.flags.enable_camera_compat_for_desktop_windowing
Fixes: 381183179
Test: atest WmTests:CameraCompatFreeformPolicyTests
Test: atest WmTests:ActivityRecordTests
Change-Id: Id736389316c9e1eed837a0f60d9428a48dac2e2b