autotest(wifi): Use WPACliProxy for all wpa_cli invocations.

There are a bunch of different places where we invoke wpa_cli commands
in the server tests:
1. On the server side in wifi_client.py (|WifiClient.reassociate|).
2. On the client side in shill_xml_rpc_server.py
(|ShillXmlRpcDelegate.request_roam|) and wifi_proxy.py
(|WifiProxy.clear_supplicant_blacklist|).

Changes:
1. Move all of these invocations to one central place i.e WifiClient on the
server side and remove the associated XML RPC methods.
2. Invoke all wpa_cli commands using the WpaCliProxy instance
initialized in WifiClient. WpaCliProxy was formerly being used as a Shill XML
RPC server substitute on android devices. Now we're going to use this
class for running wpa_cli commands for Android, Brillo and Cros testing.

BUG=b:25432676
TEST=`test_that --board=$board $host network_WiFi_Reassociate` on both
Dragonboard and Cros device. (Tests |WifiClient.reassociate| method)
TEST=`test_that --board=$board $host network_WiFi_Prefer5Ghz` on Cros
device. (Tests |WifiClient.clear_supplicant_blacklist| method)
TEST=`test_that --board=$board $host network_WiFi_PMKSACaching` on Cros
device. (Tests |WifiClient.request_roam| method)

Change-Id: I339260401b882b4142c28e24df15038aca9c17f0
Reviewed-on: https://chromium-review.googlesource.com/310229
Commit-Ready: Roshan Pius <rpius@chromium.org>
Tested-by: Roshan Pius <rpius@chromium.org>
Reviewed-by: Christopher Wiley <wiley@chromium.org>
Reviewed-by: Roshan Pius <rpius@chromium.org>
4 files changed