Remove redundant IMM#mServedInputConnection.

Following two fields have basically the same lifetime.
 - InputMethodManager#mServedInputConnection
 - InputMethodManager#mServedInputConnectionWrapper
Hence we do not need to maintain both of them.

This is a preparation CL for Bug 25332806 and does not change any
user-visible behavior.

Bug: 25332806
Change-Id: I1181e067aa5bedbdf0c7ec1bcec479257aea511c