Always install system_ext manifest.

Add a default system_ext manifest so that it is always installed
even when SYSTEM_EXT_MANIFEST_FILES are not defined.

Bug: 137712494
Test: m check-vintf-all -j

Change-Id: I0d2c967ff3f3d0b64c71f7b3dec7aef5a422ea8c
diff --git a/vintfdata/system_ext_manifest.default.xml b/vintfdata/system_ext_manifest.default.xml
new file mode 100644
index 0000000..b874bf0
--- /dev/null
+++ b/vintfdata/system_ext_manifest.default.xml
@@ -0,0 +1,2 @@
+<manifest version="1.0" type="framework">
+</manifest>