ref #729

As per getsentry/develop#931 we can add the abnormal_mechanism mechanism field to sessions to indicate to Sentry if a session was errored because of ANR. This allows the product to calculate ANR rate.

ref getsentry/sentry-java#2475

Abnormal mechanism can be either one of anr_foreground or `anr_background as per https://github.com/getsentry/sentry/blob/bf6651cb323c6c45eab2b31d4078b21756dbdff6/src/sentry/sentry_metrics/indexer/strings.py#L139-L140