Firebase-6.15.0

This release includes updates for:

• Firebase Core v6.6.0 (a.k.a 6.15.0)
• Firebase InstanceID v4.3.0 (a.k.a 6.15.0)
• Firebase Analytics v6.2.1 (a.k.a 6.15.0)
• Firebase AdMob v7.53.1 (a.k.a 6.15.0)
• Google SignIn v5.0.2
• Firebase Auth v6.4.2 (a.k.a 6.15.0)
• Firebase Crashlytics v3.14.0.2
• Google Cast v4.4.6
• Google Maps v3.7.0
• Google Places v3.7.0
• Firebase ABTesting v3.1.2
• Firebase RemoteConfig v4.4.6
• Firebase Performance Monitoring v3.1.9 (a.k.a 6.15.0)