apex_sets is added to apexkeys.txt

apex_sets is a new module type that can be used to deliver pre-signed
APEXes, which previously could be done only via prebuilt_apex.

Soon gnow understands apex_sets module types and emits the signing info
of the modules to apexkeys.txt

Bug: 158729168
Test: m
Change-Id: I9507375342ec053309660d94c931a79bf4f21218
diff --git a/apex/prebuilt.go b/apex/prebuilt.go
index 03266c5..bf574dc 100644
--- a/apex/prebuilt.go
+++ b/apex/prebuilt.go
@@ -278,6 +278,10 @@
 	return a.prebuilt.Name(a.ModuleBase.Name())
 }
 
+func (a *ApexSet) Overrides() []string {
+	return a.properties.Overrides
+}
+
 // prebuilt_apex imports an `.apex` file into the build graph as if it was built with apex.
 func apexSetFactory() android.Module {
 	module := &ApexSet{}