Removes left/right buttons from bottom area.

The left/right button/affordance functionality is dead code for a deprecated
feature. This CL removes that code from KeyguardBottomAreaView and
cleans up usages. It also deletes a couple of now-unused classes.

The greater context for this change is the desire to refactor/modernize
the KeyguardBottomAreaView. The smaller the API surface this class
supports, the easier the refactor will be.

Test: Manually verified that the lock screen still seems to behave the
same way as it did before this change. Specifically, made sure that the
Wallet and Home Control buttons are still visible when configured and
work in the normal way.
Bug: b/235403546

Change-Id: Ib5104fafa5aae07098cd7c4d51d3e11b9e335f03