Remove dependency on vncviewer since it's not available on LUCI worker, set window handle manually instead.

This approach should ensure that driver.current_window_handle
property is available on ChromeOS tests. On the local runs
spawning vncviewer process helped with that, however this
binary is not available on LUCI worker and it's not easy to
make it available. Instead I found a workaround which allows
us to set the current window manually.

Change-Id: I795e28dfc9c68d5349cd51e909df25aaa05a98b9
Bug: 413580167
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6487689
Reviewed-by: Luc Nguyen <lucnguyen@google.com>
Commit-Queue: Arkadiusz Tomczak <artomczak@google.com>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1453101}