Add Keyguard DPM state change callback

DevicePolicyManager can change the security mode
(ie: pin/pattern/password/none), so on DPM state changes,
update SecurityContainer so that the Bouncer will be in the
correct security mode if there are any changes while the device
is already on the keyguard.

Prior to this CL, the security mode would eventually be reset when
the device went to sleep & woke back up; however, this issue
was causing flaky & slow tests.

Test: manual
  From home screen: adb shell locksettings set-pin "1234"
  From lock screen: adb shell locksettings clear --old "1234"
  On lock screen: adb shell wm dismiss-keyguard
  Observe & expect: keyguard is dismissed (previously bouncer would show)
Fixes: 223440441
Change-Id: I22281dcc85d75509f1a754d57359d5f862e3d8d0