Add a lastValidated bit and use it when reporting capabilities.

When we switched the way the status bar determines if a
connection is validated from using INET_CONDITION_ACTION
broadcasts to calling getDefaultNetworkCapabilitiesForUser(),
the statusbar stopped displaying ! when a network stopped having
working Internet connectivity. This is because the validated bit
is never set to false once a network is validated.

Fix this, hopefully temporarily, by introducing a new validated
bit that does go back to being false when a network no longer
has working connectivity, and use that bit in
getDefaultNetworkCapabilitiesForUser().

Bug: 18777225
Change-Id: I991c068be50252391d0e64c647fcf2e053dc82f9