Export release flag types to make/soong

And use the types to appropriately type selects on the release
variables.

Bug: 323382414
Test: Presubmits
Change-Id: Ide7eca95662caaa7b4be42e20399d9fcd7fed35f

android/variable.go

@@ -492,6 +492,8 @@ type ProductVariables struct {
 
 	BuildFlags map[string]string `json:",omitempty"`
 
+	BuildFlagTypes map[string]string `json:",omitempty"`
+
 	BuildFromSourceStub *bool `json:",omitempty"`
 
 	BuildIgnoreApexContributionContents *bool `json:",omitempty"`