Check for missing values in mIsolatedOwners

This change ensures that when we're looking up the owner of an isolated
uid, we correctly block access when we don't find it. The default prior
to this was 0, essentially giving root visibility to an isolated calling
process.

As a result, we now log error in such a scenario. This should help in
root causing the underlying condition.

Bug: 180418767
Test: manual; run webview apps and confirm presence of logs during startup

Change-Id: I536f72d4ed53f316ba5b4bc98c6eb7f9ba0a62b8