Explicitly wait for the WebView Zygote.

There is a time-window during which the WebViewZygote service has
started, but can't be connected to. To avoid crashing during this
time-window we add a retry-loop in this CL to explicitly initiate a
connection to the Zygote.

Bug: 36687066
Test: gts-tradefed run gts --module GtsWebViewHostTestCases
Change-Id: Ia58a3342cd4e149621ba6b4aaf926e7b53c06d8f