Settings: catch activity not found exception

if CustomLauncher3 is not installed or disabled,
Settings will crash when trying to launch the keyguard wallpaper picker.

do not call activity via xml, call it in code and wrap the
startActivity call around a try-catch

to reproduce:
*) connect your phone with developer mode enabled and issue:
	adb root
	adb shell pm disable com.android.launcher3
*) go to settings and try to set a lockscreen wallpaper

Change-Id: I9f5b78e566887a28542f42f981258c8423485606