override_apex module type is used to override existing apex module with
certain properties overridden. Currently, only the 'apps' property is
overridable.
Bug: 144338929
Test: m
Change-Id: Ic050b062093cda29ce78126cc92dd6097647f7db
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