On device lockdown, always show the keyguard

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
Observe: user is brought to the keyguard, primary auth is required
to enter the device. After entering credential, the device is still in
app pinning mode.

Test: atest KeyguardViewMediatorTest
Test: manual steps outlined above
Bug: 218495634
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:100ae42365d7fc8ba7d241e8c9a7ef6aa0cdb961)
Merged-In: I9a7c5e1acadabd4484e58573331f98dba895f2a2
Change-Id: I9a7c5e1acadabd4484e58573331f98dba895f2a2