Add a prod var to override package names.

Currently only java/app.go is affected by package name overrides. When
the var is used, the corresponding module's install APK name is changed
to the overriding name.

Bug: 122957760
Test: app_test.go + TreeHugger
Change-Id: Ie522da3d7280970d740d779cf2694560feae6180

android/variable.go

@@ -271,6 +271,7 @@ type productVariables struct {
 
 	ManifestPackageNameOverrides []string `json:",omitempty"`
 	CertificateOverrides         []string `json:",omitempty"`
+	PackageNameOverrides         []string `json:",omitempty"`
 
 	EnforceSystemCertificate          *bool    `json:",omitempty"`
 	EnforceSystemCertificateWhitelist []string `json:",omitempty"`