Allow custom impression/dismiss thresholds in FirebaseInAppMessagingDisplay

[bartek-wesolowski]

What feature would you like to see?

I'd like to be able to set custom values as impression and dismiss thresholds in FirebaseInAppMessagingDisplay. Right now they are final:

  static final long IMPRESSION_THRESHOLD_MILLIS = 5 * 1000; // 5 seconds is a valid impression
  static final long DISMISS_THRESHOLD_MILLIS =
      20 * 1000; // auto dismiss after 20 seconds for banner

How would you use it?

I would set different thresholds, e.g. I wouldn't require 5s of displaying a message to count as an impression.

[google-oss-bot]

I couldn't figure out how to label this issue, so I've labeled it for a human to triage. Hang tight.

[bartek-wesolowski]

I implemented the necessary changes in #4732