adb: clean up handle_host_request.

Previously, we were returning the result of SendOkay/SendFail in a few
places after handling a host request, which is incorrect for two
reasons. First, the return type of SendOkay/SendFail is bool, and
handle_host_request was expected to return 0 on success. Second, we
don't care if the SendOkay fails; if we got to that point, we're done
with the request, regardless of whether we succeeded to report our
result. The result of this was a spurious failure result reported after
the initial result, which was ignored by the adb client.

Test: manually straced adb server
Test: python test_adb.py
Test: python test_device.py
Change-Id: I7d45ba527e1faccbbae5b15e7a0d1557b0a84858