Remodel Network Selection

    There are two main changes in this CL:
    (1) Add user selection preference support. If user
    explicitly chooses a network, framework will remember
    it and consider user has a higher preference of this
    network over all other visible networks.

    (2) Seperate user set BSSID and framework chosen BSSID.
    The BSSID under WifiConfiguration is set by user and is
    not allowed to be overrided. The BSSID under Network
    selection status is the best BSSID chosen by the
    framework to associate with.

BUG=26012244
BUG=26721725

Change-Id: Iece91d20b139ef1c6aec05828c734d1706d81007