Add credential verification command to locksetting.

This is useful when we need to unlock the user in a test.
Currently credential verification is done for any command
(e.g. "clear") when "--old" parameter is provided, so this
CL doesn't add anything new really, just makes it official.

Test: adb shell locksettings set-pin 1111
Test: adb shell locksettings verify --old 1111
Bug: 36608972
Change-Id: I3fdd6a309479739567622285945d17bb3acf972e