Create a singleton all_apex_contributions module type

This will be a container for the the apex_contributions
selected using build flags. This module will be used to query the state of
selected apex contributions instead of a global that can be mutated by
anyone.

It will set a provider containing metadata for source vs prebuilts
selection. To reduce the overhead of a new mutator, this will be done in
the existing `prebuilt_select` mutator.

It will validate that there are no dups (`foo` and `prebuilt_foo` cannot
be both selected)

Bug: 308174923

Test: go test ./android

Change-Id: Ie42999a71f35d70e0e977f5ab07ce451608d9f35
diff --git a/Android.bp b/Android.bp
index 63de015..b1db8e9 100644
--- a/Android.bp
+++ b/Android.bp
@@ -130,3 +130,8 @@
     // Currently, only microdroid can refer to buildinfo.prop
     visibility: ["//packages/modules/Virtualization/microdroid"],
 }
+
+// container for apex_contributions selected using build flags
+all_apex_contributions {
+    name: "all_apex_contributions",
+}