Clean-up ExpandableController

This CL cleans up ExpandableController so that we don't expose State
properties and instead expose normal properties (invisibly backed by a
State). This is because it's general good practice to not expose State
parameters, and this old code was adhering to this practice.

Bug: 285250939
Test: N/A
Flag: EXEMPT simple clean-up
Change-Id: If0c48e56cadf43b01f9909a2642c37ac72e50d63