Make mUserAspectRatio final

When user changes aspect ratio via Settings, the app is always killed
and user will have to open it again to initialise the new setting. Since
the aspect ratio will not change during the lifecycle of the activity,
we make it final.

Flag: EXEMPT refactor
Fix: 315140179
Test: atest AppCompatAspectRatioOverridesTest
      atest AppCompatOrientationPolicyTest
      atest SizeCompatTests
Change-Id: I3ed20c7473f3b142d63d1a6cd3a382004bd61d35