Omit icons from bulk SessionInfo calls.

Calls that return List<SessionInfo> can end up returning hundreds of
sessions, each of which include a "Bitmap appIcon" object.  Since we
quickly smash the entire list through a ParceledListSlice, we risk
exhausting FDs for all the ashmem regions.

To mitigate this, we omit these icons from the bulk calls, and then
fetch individual icons on-demand when explicitly requested.

Test: create 2k sessions and verify system doesn't croak
Bug: 38021844
Change-Id: I932073defe5b9b34a636818a69009d49060215fb