Camera: Modify CameraCharacteristics documentation about static keys.

Beginning in Android TM, we have relaxed the requirement that
characteristics keys be static. SENSOR_ORIENTATION may change based on
the state of the device. Re-word the parts of the documentation making
a guarantee that key values don't change.

Bug: 271845957
Test: m sdk
Change-Id: Ib0e2e6113afdc01ac6c29822a6ab12c19c3fd9f0