Extract ExpandableControllerImpl out of Expandable

This CL extracts ExpandableControllerImpl and introduces an
Expandable(ExpandableController) overload so that people can create a
controller before the Expandable that it will control. This allows to
trigger the animation from outside the Expandable. See [1] for an
example.

Outside of that new Expandable overload and
rememberExpandableController(), this is a pure refactoring.

[1] https://drive.google.com/file/d/1ghz2Zh9Syf4k2nHXddSrZxcl8Es6pA9m/view?usp=sharing

Bug: 230830644
Test: Manual
Change-Id: Iaa39a2c75e8a49770eaf5d8ce66d5ac562d9320c