DO NOT MERGE Fix for syncs being dropped when appIdle is on

Syncs were being dropped when appIdleMode was on for
an app. This CL backs off the sync instead of dropping
it. When the app becomes non-idle, backoff is cleared
and the sync is performed.

Bug: 26355386
Change-Id: I2040dfd847011d3ca902e66a8cd52b2a429177c1
(cherry picked from commit 2c051498)