Separate apex selection from apex.Prebuilt

This dedups the apex selection (which was previously being done by both
the apex.Prebuilt module type and the Deapexer module type) by
separating it out into its own module.

The apex.Prebuilt module retrieves the selected apex from its
selected_apex property which is simply initialized with
":<selector-module-name>".

The Deapexer module retrieves the selected apex it should use from its
src property which is initialized in the same way. That makes it easy
for the Deapexer module to be reused by the apex_set in a similar way.

Bug: 181267622
Test: m droid
Change-Id: I90cfb55698d35a97dcf28b95afcb1f58584bc90c