Fix setSecondaryLockscreenEnabled flag behavior.

setSecondaryLockscreenEnabled now checks for the flag and
if mSupervisionManagerInternal is not null to call into the
SupervisionService. This allows for the old behavior to remain intact.

This fixes CTS breakage when both flags are not enabled.

Test: atest -c 'CtsDevicePolicyManagerTestCases:com.android.cts.devicepolicy.MixedDeviceOwnerTest#testSecondaryLockscreen'
Bug: 379041099
Bug: 379976391
Bug: 383360361
Flag: android.app.admin.flags.secondary_lockscreen_api_enabled
Change-Id: I84e14508184f8e249cdab1e755d377bccd0272ee