Export OsTypeList through a function.

This is a cleanup CL to make the OsTypeList accessor consistent with
ArchTypeList by creating a function that creates a copy of the
underlying slice.

Test: TH
Change-Id: I346c5c3937bbdb8a373e4d7135b896a5274cc1f1
diff --git a/sdk/update.go b/sdk/update.go
index 828c7b6..522a888 100644
--- a/sdk/update.go
+++ b/sdk/update.go
@@ -1350,7 +1350,7 @@
 // Compute the list of possible os types that this sdk could support.
 func (s *sdk) getPossibleOsTypes() []android.OsType {
 	var osTypes []android.OsType
-	for _, osType := range android.OsTypeList {
+	for _, osType := range android.OsTypeList() {
 		if s.DeviceSupported() {
 			if osType.Class == android.Device && osType != android.Fuchsia {
 				osTypes = append(osTypes, osType)