Simplify UpstreamNetworkMonitor callback handling

In the callback post a lamba to the target state machine's handler
that does the processing we need before sending a notification to
the state machine.

No semantic change, just a bit cleaner.

Test: as follows
    - built (bullhead)
    - flashed
    - booted
    - runtest frameworks-net passes
    - tested basic wifi-to-mobile tethering (no DUN)
Bug: 32163131
Change-Id: I07e1b510c1ebaa5dffd42a3f16ba96e961cb58f1