Capture first display for multi-display devices

When no display id is explicitly specified, the first display is
captured, and a warning message is printed out. This is to make
the utility to be backward compatible with previous versions so
tools depending on it continue to work.

Bug: 241285477
Test: manual, run screencap command to verify
Change-Id: I713e1e5c625fd80ecb5587e0b2313f58baaffd40