Ensure no starting window leak in WMShell before adding.

Before adding the snapshot start window, remove any existing window
at the same task if any. Also make sure if starting window was create
failed, the window should be remove after it been added.

Bug: 185318171
Test: manual test without ag/14308714
Change-Id: I5de7d54907bc0318c6d979c44ca8e64a5fcea062