Fix bug 6688060: breakage in AndroidCharacter.mirror

A code cleanup inadvertently changed the semantics of the function,
causing it to return immediately when the first character is mirrored.
This causes CTS test breakage (in
android.text.cts.AndroidCharacterTest). This change restores the earlier
behavior and causes the test to pass.

Change-Id: I68cfb75f1db16eaa72e569dc48fbd0dee06f2de8