Add ability to modify credential via shell command

Test: adb shell locksettings set-pattern 1234
Test: adb shell locksettings set-pin --old 1234 1234
Test: adb shell locksettings set-password --old 1234 1234
Test: adb shell locksettings clear --old 1234
Test: runtest frameworks-services -c
com.android.server.LockSettingsShellCommandTest

Change-Id: I8f541effc7eab0d7453cd9a9b46c280a6425e258