Extract ApexFileProperties from apex PrebuiltProperties

The properties and logic to select the appropriate arch specific src
property will need to be shared between the 'prebuilt_apex' module type
and an upcoming 'deapexer' module type to which the `prebuilt_apex`
will delegate responsibility for exporting the '.apex' file's contents.
This refactoring extracts them into the new ApexFileProperties struct
for reuse.

Bug: 171061220
Test: m nothing
Change-Id: Iac321a28afc469e885ee5b19ad33fecd94117236