mapclient: use symbolic constant for downloading attachments

In MapClient code, a boolean can be specified when downloading a message
from the Map Server to indicate whether attachments should be downloaded
as well. These booleans are currently directly coded throughout the
code; replace with a symbolic constant.

Bug: 323412264
Test: atest MapClientStateMachineTest
Flag: EXEMPT, mechanical refactor
Change-Id: I888df35444de96842184a0506749b63cb07bc9d5