Re-structure StackScrollerDecorView.

This is not a behavior change, just moving some code around and renaming
some things to make them more readable and consistent.

More specifically:
- re-ordering methods a bit so that related ones are in the same place,
  and the order and parameter naming is consistent
- renamed things that were called Runnable without actually being a
  runnable, and turned a couple of consumers into methods for simplicity
- made isSecondaryVisible/setSecondaryVisible protected, because it
  tends to be confusing when you don't know what the secondary view
  actually is (e.g. in the footer, it's the Clear all button); it makes
  sense for the child class to define these methods more clearly

Bug: 293167744
Test: presubmit + manually checked that the footer view, empty shade and
silent section behave normally
Flag: NONE

Change-Id: I3b7d70a97472493fc00a741848e0aa36e62edc88