ConnectivityManager: allow custom Handlers

This patch adds overloaded version of registerDefaultNetworkCallback
registerNetworkCallback, and requestNetwork with an additional Handler
argument that is used for running the caller provided NetworkCallback.

It also clarifies the documentation of the existing methods that
implicitly uses the internal singleton ConnectivityThread about which
internal Handler is used for running NetworkCallbacks.

Test: build, flashed, booted device
Bug: 32130437
Change-Id: Iae15f81e47e2dc0355baf2f2c1679b77e56af299