Show RemoteInput images in notification history.

For cases where we can't show images, show "Image inserted" text instead. This will happen for work profile notifications, and for notifications that don't use MessagingStyle.

Test: Updated unit tests for NotificationRemoteInputManagerTest to use new RemoteInputHistoryItem structure, and to ensure support for URIs and mime types. Manual testing to ensure images appear in notification history for MessagingStyle notifications under the primary user, and that "Image inserted" appears for work-profile and non-messaging notifications.

Change-Id: I254b579aa710337039a7def6880f14a61bbd4d53