Default apex's compile_multilib to "first" in bp2build

In Soong, decodeMultilib, used to get multilib to determine the dep variations, return "first" if defaultMultilib is set to "common". apex sets defaultMultilib to "common" which means equivalent compileMultilib in bp2build for apex should be "first" (See new Soong unit tests for more context).

This CL fixes bp2build for apex to be more correct.

Bug: 251559512
Test: go tests
Change-Id: Id1cb4407980fc1fab91822c81326f37fb4adfa0a