Introduce immediate CONNECTIVITY_ACTION variant.

New broadcast that is dispatched immediately after connectivity
changes have been handled by ConnectivityService, bypassing any
applicable CONNECTIVITY_CHANGE_DELAY.

Also protect CONNECTIVITY_CHANGE broadcasts, since they should only
be sent by system.

Bug: 5198167
Change-Id: I75f1fb44b21da1879f0ab960bcaa481126d70fde