Move explicit mobile request handling into UpstreamNetworkMonitor

Also:
    - some minor typo fixes
    - some minor style fixes
    - variable renames for clarity

Test: as follows
    - built (bullhead)
    - flashed
    - booted
    - runtest frameworks-net passes
    - regular (no DUN) wifi tethering to mobile works
    - USB tethering with switching between mobile and wifi works
Bug: 32163131
Change-Id: I113536f24436f0b962f402f3b37386e29468dc0a