Don't spam the Email app 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

Bug: 5433427
Change-Id: I25a20279c065bfc1b22c5e2633fc465109cca495
1 file changed