Change FalsingManager to an interface for easier swapping.

This is a refactor. It touches a lot of files, but zero functionality.
The primary change is changing FalsingManager.getInstance() into
FalsingManagerFactory.getInstance(); chaning FalsingManager into an
interface, and then turning the existing FalsingManager into
FalsingManagerImpl, an implementation of that interface. Other changes
are merely references to those classes.

Bug: 130256776
Test: atest SystemUITests
Change-Id: I5d64a7673e4efc554105dd841b27807361ed3828