New AndroidMk authoring system based on entry map.

The new system collects all Android.mk variable assignments using a map
and writes them to io.Writer. Compared to the previous system, which
directly writes all entries to buffers, this new system is more robust
and test-friendly.

Test: Built without prebuilt_etc.go change and diffed the mk output.
Test: prebuilt_etc_test.go
Change-Id: Idd28443d129ff70053295015e69328a8fa3eca47