split apex.go

apex.go is too big. Separate the build rule and android.mk generation
logic into builder.go and androidmk.go, respectively. prebuilt_apex is
moved to prebuilt.go as well.

No refactoring has been made other than the splitting.

Test: m
Change-Id: I839ab0a1ba2b70ce82d98ac1fa8e3534808b5fd3