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
Merge tag 'tags/android-4.4.2_r1' into HEAD

Android 4.4.2 Release 1

# gpg: directory `/home/psxdeveloper/.gnupg' created
# gpg: new configuration file `/home/psxdeveloper/.gnupg/gpg.conf' created
# gpg: WARNING: options in `/home/psxdeveloper/.gnupg/gpg.conf' are not yet active during this run
# gpg: keyring `/home/psxdeveloper/.gnupg/pubring.gpg' created
# gpg: Signature made Tue 10 Dec 2013 06:59:18 PM CET using DSA key ID 9AB10E78
# gpg: Can't check signature: public key not found