Make extractors separate libraries

instead of being integrated into libstagefright

Test: thoroughly
Change-Id: I11cdfde6d2daf248c8e0f99237db1c74be260eb0
diff --git a/media/extractors/Android.bp b/media/extractors/Android.bp
new file mode 100644
index 0000000..e8176cf
--- /dev/null
+++ b/media/extractors/Android.bp
@@ -0,0 +1,3 @@
+subdirs = [
+    "*",
+]