Always specify underlying enum type.

Bug: 33197891
Test: compiles
Change-Id: I3d4822ea9a665708302d3bd1460425fddf1dcda9
diff --git a/audio/2.0/IDevicesFactory.hal b/audio/2.0/IDevicesFactory.hal
index a4e6956..f1dc7d3 100644
--- a/audio/2.0/IDevicesFactory.hal
+++ b/audio/2.0/IDevicesFactory.hal
@@ -22,7 +22,7 @@
 interface IDevicesFactory {
     typedef android.hardware.audio@2.0::Result Result;
 
-    enum Device {
+    enum Device : int32_t {
         PRIMARY,
         A2DP,
         USB,