Make NetworkAgent stop subclassing Handler.

Subclassing Handler is not appropriate for a system API because
it is an implementation detail and allows users of this class to
post messages to the handler in ways that allow inappropriate
access to internals that aren't part of the API contract.

Also fix some lint errors.

Test: builds
Bug: 138306002
Change-Id: I79478ceff6bbcae879d1025098d177de0d15dbee