Deprecate FLAG_DISMISS_KEYGUARD

Since we now have an explicit dismiss method the flag is only
dangerous for falsing. The behavior will be migrated in the
following way:

- Insecure: Treat as FLAG_SHOW_WHEN_LOCKED
- Trusted: Actually dismiss Keyguard.
- Secure: Show bouncer.

We also restore the behavior to not allow dismissing the Keyguard
while it is occluded, which was the case in Nougat.

Test: cts/hostsidetests/services/activityandwindowmanager/util/run-test
android.server.cts.KeyguardTests

Test: cts/hostsidetests/services/activityandwindowmanager/util/run-test
android.server.cts.KeyguardLockedTests

Test: Insecure: Launch FLAG_DISMISS_KEYGUARD activity, make sure
Keyguard is occluded.

Test: Secure: Launch FLAG_DISMISS_KEYGUARD from SHOW_WHEN_LOCKED, make
sure bouncer is shown.

Test: Trusted: Launch FLAG_DISMISS_KEYGUARD from SHOW_WHEN_LOCKED, make
sure Keyguard gets unlocked.

Test: Trusted: Launch FLAG_DISMISS_KEYGUARD, lock screen, make sure
Keyguard is not dismissed/occluded.

Change-Id: I0d1ec9397a83975adb065c6cb81bf23b08c55395