Extract Expandable related functions to be used by ag/31718343

This CL simply extracts some functions from Expandable (and related
classes) that will be reused by ag/31718343. This is a pure refactoring
meant to make the parsing/review of ag/31718343 easier.

Bug: 285250939
Test: N/A
Flag: EXEMPT simple code extraction
Change-Id: Ib4541b474323d9c68433560a5bda193f45ff860d