Pause map's location manager on sleep

[friedbunny]

• Factors out the location manager start/stop code into new methods.
• Calls these new methods on wakeGL and sleepGL, if necessary.
• Only asks for location permissions if not already granted/denied.

Stuck this on v3.1.0, but we can move it off if this proves bothersome or controversial.

/cc @1ec5 @boundsj

[1ec5]

This would be a lot simpler if the logic for enabling and disabling the location manager were consolidated in one place, since multiple factors can determine whether the location manager is paused. Take a look at -validateDisplayLink as an example of this validation pattern.

[friedbunny]

@1ec5 This has been refactored to a single validation-style method in f6f91e0. Can't think of any edge cases, but maybe you can?

[friedbunny]

→ #4034 for inclusion in v3.1.2.