Add a boolean value on whether to use direct channel callback for all ios versions

[charlotteliang]

Add a new boolean value that control whether to use direct channel callback for all iOS versions. This partially resolves the issue #1409.

We are separating the callback between apns and direct channel. Right now for iOS 10 and above, direct channel message delivered at FIRMessagingDelegate, while for iOS 9 and below, they were delivered at UIApplicationDelegate. To not break existing behavior, if user likes to receive direct channel callback using FIRMessagingDelegate instead of UIApplicationDelegate for all ios versions, they can now set the boolean value useMessagingDelegateForDirectChannel to true.

[paulb777]

Please fix travis issue:

$ ./scripts/check_whitespace.sh
Firebase/Messaging/Public/FIRMessaging.h:434:#pragma mark - FCM Direct Channel 
ERROR: Trailing whitespace found in the files above. Please fix.

[paulb777]

Can the new flag be both set via an API and an Info.plist setting?

[paulb777]

Remove ifdef. It should be created by copybara

[paulb777]

One more change needed. I noticed 'GoogleUtilities/Environment' is missing from the podspec dependencies in https://github.com/firebase/firebase-ios-sdk/blob/master/FirebaseMessaging.podspec. It's working because of transitive dependencies, but it should be added to make the dependency explicit.

[paulb777]

LGTM on travis green.

[morganchen12]

LGTM with one optional nit.