Downscale oversized MessagingStyle avatars

Notification API downscales oversized icons in all formats except in MessagingStyle. This seems to be an oversight that can cause massive memory usage.
This downscales avatars to a reasonable size if they're too big.

Bug:193720474
Bug:221890932

Test: atest NotificationTest
      Built a test app with 4000x4000 avatar icon.
      Memory usage went from 250MB to ~170KB per notification.
Change-Id: I35377cdb5d61ee6e23fccc2bffb5d3aec6ca2b7d