For platform APEXes, file_contexts should point a file under
/system/sepolicy.
Bug: 144732805
Test: m
Change-Id: Ib2d5db715bbebc80a6178d1c42e387b268cc4a0d
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