Add feature flag to Font Scaling Tile

1. Return the flag for isAvailable in the tile
2. Update tests to verify the dialog would showup

Bug: 242326166
Test: manually test the feature flag
Test: atest FontScalingTileTest
Change-Id: I5f44abfe6d37d0d6904195864a0ad7eafb0aedf5
2 files changed