move image decode into background thread

Fixes strict mode violations. Now it also handles arbitrarily
large full-size images, which is all EAS/IMAP may offer.

Use contentUri when thumbnailUri is not populated (as is the
case with EAS/IMAP).

Bug: 6195064
Bug: 6389109
Change-Id: Ifcc2062d35e77c0fca4e6a568488ab556ba0da08
4 files changed