DO NOT MERGE: Don't spam Email with attachment load progress

* I observe that the read() can return with as little as 100
  bytes read; this can lead to lots of spamming via the binder
  interface.
* Limit callbacks based on both percentage read and bytes read
* Clean backport of I25a20279c065bfc1b22c5e2633fc465109cca495

Bug: 5433427
Change-Id: I513a1e55ccdb1781b094dddad26ef6c64693002d
1 file changed