recovery: Hide unmountable volumes from selection

* In volume manager, check if new volumes are mountable.

* Check volumes for mountable for inclusion into update list.

* Erase unmountable volumes from volumes vector for consistency with
  the item array.

Change-Id: I89ff6cc05a93afffe5e46b24d70fc368bccaf020
diff --git a/volume_manager/EmulatedVolume.h b/volume_manager/EmulatedVolume.h
index ac40b6f..66421cc 100644
--- a/volume_manager/EmulatedVolume.h
+++ b/volume_manager/EmulatedVolume.h
@@ -55,6 +55,8 @@
     unsigned long mFlags;
     std::string mFsOptions;
 
+    bool detectMountable() override;
+
     DISALLOW_COPY_AND_ASSIGN(EmulatedVolume);
 };