Set the default screenshot delay to 0ms

Bug: 165267251
Test: manual -- ensured that the time needed to hold is 0ms if the
screenshot_keychord_delay debug value is not set, and that the
delay can still be changed using

adb shell device_config put systemui screenshot_keychord_delay <ms>

Change-Id: Iab989ecf14ef379658130adbced241e084554e63