Update LockSettingsCmd's showUsage method to defer to calling

"locksettings help".

This removes the duplication and also fixes the
current behavior showing the incorrect documentation for "locksettings
verify" which does *not* log in the user.

Test: adb shell locksettings
Test: adb shell locksettings clear --old 0000 (to verify other commands
still work)

Change-Id: I429288a9e4280d41213e1c0fd8234d3666888abc