Simplify deapexer support

Uses the apex relative path to the file as the identifier that is used
to obtain the path to the corresponding file extracted from the apex.
That is instead of a special constructed string id.

Bug: 177892522
Test: m nothing
Merged-In: I5dc77c8fb272bac289b8891d1eac801e541af1f5
Change-Id: I5dc77c8fb272bac289b8891d1eac801e541af1f5
(cherry picked from commit b4bbf2ca)