Mtp: MtpService TOMBSTONE continue happen when do usb otg plug/unplug test.

Call new interface in MtpDatabase to delete global reference
in order to avoid GREF issue. The patch depend on MtpDatabase class.

Note: This patch is related to a change in platforms/frameworks/base

Change-Id: Id980f669c56461f1e4b48f52d416f3651f15589a
Author: Bo Huang <bo.b.huang@intel.com>
Signed-off-by: Bo Huang <bo.b.huang@intel.com>
Signed-off-by: Shuo Gao <shuo.gao@intel.com>
Signed-off-by: Bruce Beare <bruce.j.beare@intel.com>
Signed-off-by: Jack Ren <jack.ren@intel.com>
Author-tracking-BZ: 60891
Merge remote-tracking branch 'rf/kitkat' into kitkat