Trust CA certificates added for the whole OS only

Excludes any CA certificates installed for wifi-only from being used for
anything else. Does not take effect retroactively against certs which
were already installed.

The CAs will continue to be saved to a part of the keystore accessible
by services running under WIFI_UID.

Bug: 26324357
Bug: 25780055
Change-Id: Ifeb9daf24c9f9a22b2b2daf247d5622c707c9885
