Call SF#processDisplayChangesLocked in testable SF

processDisplayChangesLocked updates rotations and should be
performed in testable SF's commitTransactionsLocked. Several
tests involving rotations had broken after the legacy frontend
flag was removed because these rotations were not being updated.
SF already calls processDisplayChangesLocked in correct
locations, but some test suites neglect to do so. These changes
are therefore isolated to just the testable SF API.

Bug: b/330785038
Test: atest ActiveDispayRotationFlagsTest
Test: atest DisplayModeSwitchingTest
Test: atest DisplayTransactionCommitTest
Test: presubmit
Change-Id: I9ca7bc70b9a5a462e623a788f7a98a738c6a3482