swarming.py bot: handle incorrect dimensions.
swarming.py bot: handle incorrect dimensions.

There's a bug where android bots can specify an invalid dimension and it is
returned as is by the server. Until the server is fixed, make the client more
resilient.

R=rmistry@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1584513002

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py