aapt2: Add signature policy to Resources.proto

Adds the new enum values for none and the new signature policy.
None is needed to ensure we don't accidentally deserialize public,
as protobuf uses the first or 0 value enum as the default.

Test: aapt2_tests pass

Change-Id: Ibcb6b961abaa7e5c048e80c4a3eaf1b6d1d421b5

tools/aapt2/Resources.proto

@@ -151,10 +151,12 @@ message Overlayable {
 // Represents an overlayable <item> declaration within an <overlayable> tag.
 message OverlayableItem {
   enum Policy {
-    PUBLIC = 0;
-    SYSTEM = 1;
-    VENDOR = 2;
-    PRODUCT = 3;
+    NONE = 0;
+    PUBLIC = 1;
+    SYSTEM = 2;
+    VENDOR = 3;
+    PRODUCT = 4;
+    SIGNATURE = 5;
   }
 
   // The location of the <item> declaration in source.