1. Change to FooterBarMixin
2. Move FooterButton to the same package with FooterBarMixin
Bug: 120805516
Test: RunSettingsRoboTests
Change-Id: Ic6937e3cbc515dd7bf877c9193932cd5800ac801
Bug: 110589286
Test: make -j56 RunSettingsRoboTests
Test: adb shell am start -a android.settings.FINGERPRINT_ENROLL still works
Test: adb shell am start -a android.settings.FINGERPRINT_SETUP still works
Change-Id: If33b557137cae7b57e4a0e906ee95032bc589436