Add overrides support for snapshots

Overrides properties will now be captured in json flag files, which will
be copied to installed vendor snapshot modules.

Bug: 216567575
Test: soong test && manual install
Change-Id: Ife5e84b126e798fba7802b9cff000c9197756cb9

snapshot/snapshot_base.go

@@ -118,5 +118,6 @@ type SnapshotJsonFlags struct {
 	CrateName           string `json:",omitempty"`
 
 	// dependencies
-	Required []string `json:",omitempty"`
+	Required  []string `json:",omitempty"`
+	Overrides []string `json:",omitempty"`
 }