Updated: always show the keyguard on device lockdown

Additionally, don't hide keyguard when it's disabled if the user has locked
down the device.

Manual test steps:
    1. Enable app pinning and disable "Ask for PIN before unpinning" setting
    2. Pin an app (ie: Settings)
    3. Lockdown from the power menu
    4. Observe: user is brought to the keyguard, primary auth is
       required to enter the device.
       => After entering correct credential, the device is still in
          app pinning mode.
       => After entering an incorrect credential, the keyguard remains
          showing and the user can attempt again up to the limit

Bug: 300463732
Bug: 218495634
Test: atest KeyguardViewMediatorTest
Test: manual
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:faaf58d3b910c388b0a7c51dc370a7ae18e7cec2)
Merged-In: I70fdae80f717712b3dfc9df54b9649959b4bb8f0
Change-Id: I70fdae80f717712b3dfc9df54b9649959b4bb8f0