Soong package structure refactoring

Give prebuilt_etc and sh_binary their own packages and split the
gigantic main Android.bp up to small, per-package ones.

(This is a cherry-pick change.)

Test: m nothing, TreeHugger
Bug: 156980228
Change-Id: I7b00cd344b9f16861f1ff39edf0029f016b853d0
Merged-In: I7b00cd344b9f16861f1ff39edf0029f016b853d0