Get rid of custom negaitve save button label

Removed the ability for an autofill provider to set
a custom string for the negative button. Also make
the old hidden APIs non-abstract to avoid runime
crashes for unimplemented methods.

Test: CtsAutoFillServiceTestCases pass

bug:37649790

Change-Id: If776d26de48382eeb064bdead5d4d5fdb0ff2e7e