timber.java

        if (BuildConfig.DEBUG) {
            Timber.plant(new Timber.DebugTree());
        } else {
            Timber.plant(new CrashReportingTree());
        }

....

    private static class CrashReportingTree extends Timber.Tree {
        @Override protected void log(int priority, String tag, String message, Throwable t) {
            if (priority == Log.VERBOSE || priority == Log.DEBUG) {
                return;
            }

            Crashlytics.log(priority, tag, message);

            if (t != null) {
                Crashlytics.logException(t);
            }
        }
    }