Android denial of service attack using class 0 SMS messages

Bug 9702645
Bug 11970403 - Change to ClassZeroActivity to singleTask

Cherry pick the change from master.

Making every AlertDialog immediately visible can lead to exhaustion
of graphics-related resources, typically memory, resulting in a
broken bufferqueue/hw renderer, and subsequent system crash.

Make ClassZeroActivity a singleTask activity, and queue incoming
messages if one is already being displayed.

Change-Id: I0aef7b857364acc404e79581ba5e503571065483