Don't emit make rules for APEX files for non-installable APEX

When an APEX is non-installable, the make rules for the APEX files in
the APEX are not emitted as they will never get installed.

androidMkForType() is refactored so that make rules for the APEX files
are created in a separate function androidMkForFiles().

Test: m checkbuild tests
Bug: 123290268
Change-Id: Ibe8817d1e9c6312fb5c6f986dced8aa3e823664a