Allow required property be conditional for release_aidl_use_unfrozen

Bug: 342336921
Test: cd build/soong/android ; go test -run TestProductVariables
Test: add product_variables.release_aidl_use_unfrozen.required to the
phony module.

Change-Id: I9b934a8fb6245b9b6fd0f2db1e67d5e2876490b2

android/variable.go

@@ -183,6 +183,7 @@ type variableProperties struct {
 		Release_aidl_use_unfrozen struct {
 			Cflags          []string
 			Cmd             *string
+			Required        []string
 			Vintf_fragments []string
 		}
 	} `android:"arch_variant"`