Convert external/libyuv to Android.bp

See build/soong/README.md for more information.

Bug: 37646797
Test: m -j checkbuild
Change-Id: I9eeafa6fe101acf59c5eb43376e5a8280158457a
diff --git a/Android.bp b/Android.bp
new file mode 100644
index 0000000..515d562
--- /dev/null
+++ b/Android.bp
@@ -0,0 +1 @@
+subdirs = ["files"]