Log upstream network suspend/resume callbacks

Also log signal strength while we're at it:

    2017-10-10T14:05:00.968 - [UpstreamNetworkMonitor] preferred upstream type: WIFI
    2017-10-10T14:06:07.654 - [UpstreamNetworkMonitor] upstream network signal strength: -62 -> -67
    2017-10-10T14:06:13.704 - [UpstreamNetworkMonitor] upstream network signal strength: -67 -> -64

Test: as follows
    - built
    - flashed
    - booted
    - runtest frameworks-net passes
Bug: 29337859
Bug: 32163131
Bug: 64976634
Bug: 67396342

Change-Id: Id6e143edc9f5b0256aa637643680f6c8dce70d90