Prebuilt APEXes are recoreded in apexkeys.txt

apexkeys.txt now correctly lists prebuilt APEXes with keys specified as
PRESIGNED.

This change also fixes a bug that non-installable APEXes are listed in
the file.

Bug: 131130235
Test: m out/soong/apexkeys.txt and check that
com.android.apex.cts.shim.apex is listed there with PRESIGNED keys.

Merged-In: Ib6d391a82864714743a1cc59cd655bea917b5073
Change-Id: Ib6d391a82864714743a1cc59cd655bea917b5073
(cherry picked from commit a41f12a6fa)

android/prebuilt.go

@@ -74,6 +74,10 @@ func (p *Prebuilt) SingleSourcePath(ctx ModuleContext) Path {
 	}
 }
 
+func (p *Prebuilt) UsePrebuilt() bool {
+	return p.properties.UsePrebuilt
+}
+
 func InitPrebuiltModule(module PrebuiltInterface, srcs *[]string) {
 	p := module.Prebuilt()
 	module.AddProperties(&p.properties)