Dedup apex/Prebuilt and apex/ApexSet

Moves common properties, fields and method into the prebuiltCommon
type.

One slight change is that this change causes the Prebuilt module to use
the outputApex when generating the AndroidMkEntries instead of the
inputApex that it used before. They are the exact same files as the
inputApex is simply copied to the outputApex. The only impact will be
on build time as some rules now depend on the cp rule being run when
they did not before and the base name of the file may be different.

Test: m droid
      - check generated out/soong/Android...mk file before and after
        to make sure the only difference is due to switching from the
	inputApex to outputApex.
Bug: 179354495
Change-Id: I8437af00d6bb7d0d339f25b3b02cd1cf67d6938a
Merged-In: I8437af00d6bb7d0d339f25b3b02cd1cf67d6938a