[3/n] Improve LetterboxConfiguration readability

Improve readability of isCameraCompatTreatmentEnabled() and
isDisplayRotationImmersiveAppCompatPolicyEnabled() in case
we need to access their build time values.

Fix: 287210791
Test: atest WmTests:DisplayRotationCompatPolicyTests
      atest WmTests:DisplayRotationImmersiveAppCompatPolicyTests
      atest WmTests:LetterboxUiControllerTest

Change-Id: I0ae4494025e1a4ba46ef91c8d4f1920b03caf333