AgentBase: Fix crash in setOnline

When setOnline tries to access the mSettings member after the latter has
been deleted we get a crash; mSettings is deleted in quit() or cleanup(),
therefore it looks like a race condition: the Agent is about to quit, but
some other code calls setOnline leading to a crash. It looks like this can
happen in more than one way, e.g. via a dbus call (from AgentInstance), or
via a sub-class of AgentBase calling setOnline directly. Use a bool member
var, modelled after how AgentInstance uses a similar logic.

BUG: 418844
(cherry picked from commit 53574eb6)