Disable source apex variant using apex_available_name

There are two versions of aosp extservices - one for Tplus and one for
Sminus. When prebuilts are active, the `contents` validation of both
variants of extservices bootclasspath fragments should be disabled to
account for BCP jar skew.

This CL adds ApexAvailableName to ApexInfo to achive that. If an
api_domain matching the ApexAvailableName has been selected in
`all_apex_contributions`, the source apex variants will be disabled.

Bug: 361771739

Test: lunch cf_x86_64_phone-next-userdebug
Test: m nothing with ag/28851886
Change-Id: I505a7ad8295c258e2eeef7f183669f7a75a1c88e

android/apex.go

@@ -87,6 +87,9 @@ type ApexInfo struct {
 
 	// Returns the name of the overridden apex (com.android.foo)
 	BaseApexName string
+
+	// Returns the value of `apex_available_name`
+	ApexAvailableName string
 }
 
 // AllApexInfo holds the ApexInfo of all apexes that include this module.