Initialize content resolver in ctor

* In ag/16337218 we moved the initialization of the
context into the ctor instead of init(), but not the
content resolver.
* When we add a callback for OverviewProxyService in
the ctor, it might call the onConnectionChanged()
before NavBarHelper#init() is called, which would lead
to use of a null content resolver in
updateAssistantAvailability()

Bug: 208261632
Change-Id: I306cdf27c7870ad72e80ef401367bb0d6dc32f68