Special-case Soong finder to look in out/api_surfaces

Add a new argument in soong_ui that will be used to gate this behavior.

This approach is expected to solve incrementality issues in multi-tree.
As part of soong's bootstrap process, all source dirs are added to deps
of out/soong/build.ninja (via globs). Since multitree_build writes to the
"source" api_surfaces directory, it changes its mtime and causes a
recompilation of out/soong/build.ninja in the subsequent invocation.

Test: TH (for single-tree)
Test: Inspected ninja files (for multi-tree)
(Run a full build)
touch out/api_surfaces/vendorapi/libc/.../math.h
orchestrator/prebuilts/build-tools/linux-x86/bin/nsjail --config
out/trees/vendor_aosp_cf_arm64_phone/nsjail.cfg --
prebuilts/build-tools/linux-x86/bin/ninja -f out/soong/bootstrap.ninja
-d explain -n out/soong/build.ninja
(ninja: no work to do)

Change-Id: Ib823163ec1153344a2f593daa8d7156c24ff5bc3