shill: wimax: Use BeginAsyncDBusCall helper.

This CL replaces WiMaxDeviceProxy::Invoke with the recently added
BeginAsyncDBusCall helper for making asynchronous DBus method calls to
WiMAX manager.

BUG=chromium:365390
TEST=Tested the following:
1. `USE='gdmwimax wimax' FEATURES=test emerge-$BOARD platform2`
2. Run platform_WiMaxPresent and platform_WiMaxSmoke test.

Change-Id: I29e9cc49bfc4d3a872b779a57c2af3fa82cf9e4d
Reviewed-on: https://chromium-review.googlesource.com/196418
Reviewed-by: mukesh agrawal <quiche@chromium.org>
Commit-Queue: Ben Chan <benchan@chromium.org>
Tested-by: Ben Chan <benchan@chromium.org>