Reduce the number of Soong invocations, and various other minor script fixes.

-  We can pass all targets at once to build-aml-prebuilts.sh.
-  Less noise from build-mainline-modules.sh.
-  Default to the same out directory in both build scripts.
-  Addressed post-submit comments on https://r.android.com/1170907.
-  Various minor cleanups.

Test: build/soong/scripts/build-mainline-modules.sh
Change-Id: Id21ef80c2334462836e217032fc5bf63b7cd04e1