Allow custom impression/dismiss thresholds in FirebaseInAppMessagingDisplay

## 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:
```java
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.