Separate metadata provider from apex contents provider

The platform_compat_config_singleton only needs access to the metadata
so this separates that method into its own interface, distinct from the
one needed by the apex. This also adds a test to ensure that the merging
still works.

Bug: 182402754
Test: m nothing
Change-Id: I5212239786810e5fc5eb99831b1122db93d1329f
diff --git a/java/Android.bp b/java/Android.bp
index 9e2db83..56cc401 100644
--- a/java/Android.bp
+++ b/java/Android.bp
@@ -75,6 +75,7 @@
         "java_test.go",
         "jdeps_test.go",
         "kotlin_test.go",
+        "platform_compat_config_test.go",
         "plugin_test.go",
         "rro_test.go",
         "sdk_test.go",