StorageVolume: Add getStorageId() accessor

This ID is used for MTP as well as per volume querying in the media provider.

Change-Id: Ic4fc986d972bd477730643f7e9450c390c0b3a4b
Signed-off-by: Mike Lockwood <lockwood@android.com>
diff --git a/services/java/com/android/server/MountService.java b/services/java/com/android/server/MountService.java
index 376d42f..347d70a 100644
--- a/services/java/com/android/server/MountService.java
+++ b/services/java/com/android/server/MountService.java
@@ -1145,6 +1145,11 @@
         } catch (IOException e) {
             throw new RuntimeException(e);
         } finally {
+            // compute storage ID for each volume
+            int length = mVolumes.size();
+            for (int i = 0; i < length; i++) {
+                mVolumes.get(i).setStorageId(i);
+            }
             parser.close();
         }
     }