Fix GpsLocationProvider.enable/disable/isEnabled

Fix the issue where GpsLocationProvider.isEnabled() returns true when it
is really false (and the other way around), when the handler hasn't
processed the enable/disable messages yet.

This can be systematically reproduced when the caller code is using the
same thread as the thread of the handler in GpsLocationProvider.

For example, this was happening in LocationManagerService.switchUser().
It would start by disabling all the providers (with
updateProviderListenersLocked()), then re-enable them in
updateProvidersLocked() only when isEnalbed()==false, which was in the
wrong state since the GpsLocationProvider.ENABLE message hadn't been
processed yet. As a result, the GpsLocationProvider was disabled upon
startup of the phone.

This is a slight problem for the enable() contract, which specifies that
getStatus() must be handled, getStatus() will be handled but might have
slighty not-up-to-date info in this case.

Bug: 8028017

Change-Id: Iff91a11cc150e9029a6db85b64a10a926e12b0ba