Access removable volumes through /mnt/media_rw

Due to permissions changes, we now need to access
the underlying filesystem of removable devices in
order to get write access.

Add internalPath to StorageVolume, and have VolumeInfo
set the field on creation.

Bug: 77849654
Test: Can write to emulated sdcard through MTP
Change-Id: I63302ecf2dd2600a1c9f3f6ab106c3695654cbaa
diff --git a/media/java/android/mtp/MtpStorageManager.java b/media/java/android/mtp/MtpStorageManager.java
index bdc8741..a36d88d 100644
--- a/media/java/android/mtp/MtpStorageManager.java
+++ b/media/java/android/mtp/MtpStorageManager.java
@@ -399,8 +399,8 @@
      */
     public synchronized MtpStorage addMtpStorage(StorageVolume volume) {
         int storageId = ((getNextStorageId() & 0x0000FFFF) << 16) + 1;
-        MtpObject root = new MtpObject(volume.getPath(), storageId, null, true);
         MtpStorage storage = new MtpStorage(volume, storageId);
+        MtpObject root = new MtpObject(storage.getPath(), storageId, null, true);
         mRoots.put(storageId, root);
         return storage;
     }