Soong: Read Android.soong.mk if skipping Android.mk

If Soong is enabled, and we're skipping an Android.mk because there is
an Android.bp file, check for an Android.soong.mk file and read that
instead. This will allow us to temporarily define modules or recurse
into subdirectories that soong does not yet support.

Change-Id: Ifdb2f0204a38a5069e53527f66ffcfb8008c11a4