Represents the Android-specific options that can be included in an admin.messaging.Message.
Collapse key for the message. Collapse key serves as an identifier for a group of messages that can be collapsed, so that only the last message gets sent when delivery can be resumed. A maximum of four different collapse keys may be active at any given time.
A collection of data fields to be included in the message. All values must
be strings. When provided, overrides any data fields set on the top-level
[key: string]: string