Convert libsoundtrigger to Android.bp

See build/soong/README.md

Test: m -j checkbuild
Change-Id: I09ab3bfc8649bda8bad66c0898e342865f8b4a92
diff --git a/Android.bp b/Android.bp
index 53f8bec..52cdada 100644
--- a/Android.bp
+++ b/Android.bp
@@ -17,4 +17,5 @@
 subdirs = [
     "camera/ndk",
     "media/*",
+    "soundtrigger",
 ]