MGLAccountManager should get its data from Info.plist entries, not class setter method calls

[1ec5]

MGLAccountManager currently forces the developer to send it -setMapboxMetricsEnabledSettingShownInApp: (optionally) and -setAccessToken: sometime at startup, in that order, before the first MGLMapView is initialized. The exact place where these setters must be called depends on how and where they use MGLMapView – whether programmatically or in a storyboard, whether in the initial view controller or some later view controller, etc.

iOS has an affordance for settings that need to be set right at launch: the Info.plist that comes with every project. MGLAccountManager should look there for the access token and in-app opt-out flag. For beta 2, we should deprecate the class setters and add code to +[MGLAccountManager initialize] (which is called as soon as MapboxGL.a loads) that looks inside the Info.plist using -[NSBundle objectForInfoDictionaryKey:].

/cc @incanus @bleege