Dialer: fix memory leak in DialpadFragment

By MAT tool, we find there exists several PopupMenu
objects after repeating operation "dial 10086 -> hangup"
several times. It's not right, there should exist only
one object of PopupMenu for DialpadFragment. This PopupMenu
object occupies 40 byte but indeed causes memory leak.
The root cause is that we will call onResume() after hangup
every time, onResume will new PopupMenu every time and set its
mDragListener to overflow image button's mOnAttachStateChangeListeners,
this listener references PopupMenu object and causes PopupMenu can not
be released by GC.

We create PopupMenu in onCreate instead of onResume to avoid
this leak.

Change-Id: Id787bc5e23f9d5f86a9fa03536e9ae901e12cb13