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/config.go

@@ -1026,6 +1026,10 @@ func (c *config) HasMultilibConflict(arch ArchType) bool {
 	return c.multilibConflicts[arch]
 }
 
+func (c *config) PrebuiltHiddenApiDir(ctx PathContext) string {
+	return String(c.productVariables.PrebuiltHiddenApiDir)
+}
+
 func (c *deviceConfig) Arches() []Arch {
 	var arches []Arch
 	for _, target := range c.config.Targets[Android] {