a11y: Remove CurrentCornerIndex

CurrentCornerIndex and Corner enum get mixed up easily. This CL removes the
CORNER_ROTATION_ORDER array and uses Corner enum values directly to make
the code cleaner.

Bug: 388847771
Test: AutoclickTypePanelTest
Flag: com.android.server.accessibility.enable_autoclick_indicator
Change-Id: I3d6c49f1d0a3d83f44e5d0b3fbcc2b4b63ec5fb3