Widen allowed UID range for webview zygote.

The webview zygote is shared for all users on the system, and so unlike
the app zygote, it can't use a single whitelisted UID range. For now,
clamp to the upper bound of the UID range, until we have a better idea.

This is still an improvement from the previous status quo, because it
will prevent setuid/setgid into system users.

Bug: 123597434
Test: builds, webview_zygote running
Change-Id: Ia975826ed5b1f20cabb46f60f5951723b1ba80c9