Handle missing migration source volume.

Users can try migrating primary storage while the current location
is missing/unmounted.  Fail gracefully instead of runtime restarting.

Bug: 21927076
Change-Id: I54b92487faf9e62d5d309734bf4c436a9259d156

src/com/android/settings/deviceinfo/MigrateEstimateTask.java

@@ -81,6 +81,11 @@ public abstract class MigrateEstimateTask extends AsyncTask<Void, Void, Long> im
         final VolumeInfo privateVol = mContext.getPackageManager().getPrimaryStorageCurrentVolume();
         final VolumeInfo emulatedVol = mStorage.findEmulatedForPrivate(privateVol);
 
+        if (emulatedVol == null) {
+            Log.w(TAG, "Failed to find current primary storage");
+            return -1L;
+        }
+
         final String path = emulatedVol.getPath().getAbsolutePath();
         Log.d(TAG, "Estimating for current path " + path);