Lockscreen Notifications: make dynamic width a device overlay setting

dynamic width is disabled by default, to enable it for a specific device, add
<bool name="config_lnDynamicWidth">true</bool>
to yourdevicetree/overlay/frameworks/base/packages/Keyguard/res/values/config.xml

Change-Id: I210b747838b70fb64e6f0835b1cb42b31c9a199d