Enable prebuilt hiddenapi CSV files.

By enabling these hiddenapi CSV files to be prebuilt, it
becomes possible to create a split build that supports
the hiddenapi encode dex step, but doesn't contain all
of the java sources needed to generate the CSV files.

Bug: 175048716
Test: m nothing
Test: new TestHiddenAPISingletonWithPrebuiltCsvFile
Test: local build without prebuilt hiddenapi
Test: local build with prebuilt hiddenapi
Change-Id: I805ea3ec05860d2df211a3985ec025bf36f0d775

android/variable.go

@@ -364,6 +364,8 @@ type productVariables struct {
 	BoardKernelModuleInterfaceVersions []string `json:",omitempty"`
 
 	BoardMoveRecoveryResourcesToVendorBoot *bool `json:",omitempty"`
+
+	PrebuiltHiddenApiDir *string `json:",omitempty"`
 }
 
 func boolPtr(v bool) *bool {