Add a flag to change the screenshot keychord delay

Allows overriding the configured keychord delay (the time it takes,
after both the power button and volume down have been pressed, for
a screenshot to be registered/taken). If not set, reads from the
config file (as before). Can be updated with

adb shell device_config put systemui screenshot_keychord_delay <n>

where <n> is the desired timeout, in ms.

Bug: 165267251
Test: manual -- tried setting multiple values, ensured that deleting
the override reverts to the configured value

Change-Id: I2f86abe0332f8072ab4f53baa55f1999aed3865a