Reconcile Expandable.kt on master/tm-qpr-dev-plus-aosp and tm-qpr-dev

This CL reconciles the content of Expandable.kt in the
tm-qpr-dev-plus-aosp branch and tm-qpr-dev. This CL being on
tm-qpr-dev-plus-aosp, this will also be merged in master.

Currently the files are different because the androidx.savedstate
library was updated in tm-qpr-dev-plus-aosp in http://ag/19946745, and
removed the ViewTreeSavedStateRegistryOwner APIs. Ideally, instead of
this CL, we would update the androidx.savedstate library in tm-qpr-dev
but I'm getting build errors in http://ag/20708667 that might take a
while to fix. Until I manage to get that CL in, I suggest we work around
it in this CL by re-introducing the ViewTreeSavedStateRegistryOwner API
used by tm-qpr-dev, which forwards calls to the API that replaced it.
That way we can unblock the few CLs like http://ag/20256703 and
http://ag/20711619 which currently have conflicts with master and
tm-qpr-dev-plus-aosp.

Bug: 262222023
Test: Builds
Change-Id: I97660c5a9a2758c5e19d037e635c21456b6085c3