[CM] Allow timeout in request network

Add (unhide) a public API which provides network requests with a
timeout. When timed-out the (newly unhidden) onUnavailable() callback
is triggered.

Note: this CL does not add a handler to the API to be consistent
with the existing APIs. There is a separate effort (b/32130437)
to update these APIs with Handlers.

Bug: 31399536
Test: unit tests and CTS (new)
Change-Id: I45ce9ada63372cb56937bb620bfbb7729e5e25d2