chore(#MagnificationThumbnail): add feature flag file and fix size not correct sometimes

We add MagnificationThumbnailFeatureFlag to encapsulates the feature flag from directly access in FullScreenMagnificationController. MagnificationThumbnailFeatureFlag is extended from MagnificationFeatureFlagBase so we can clear binder identity before access the flag. Besides, FullScreenMagnificationController can observe feature flag flips and create/destroy the thumbnail.

Originally the thumbnail size would be huge sometimes. It's because the thumbnail layout size is not updated properly based on the magnification bounds before it shows on screen. Therefore, we call refreshThumbnail to update the size when the thumbnail is just created in FullScreenMagnificationController.

Bug: 276979625
Bug: 276314641
Test: manually test with adb to flip the flag
      atest FullScreenMagnificationControllerTest
Change-Id: I1480b73454ad32f2a294bace56a285ae462119d8