Mark secondary lockscreen API as SystemApi.

It is currently not meant for use by general enterprise device admins.

Bug: 152478326

Test: atest FrameworksServicesTests:DevicePolicyManagerTest
Test: atest KeyguardUpdateMonitorTest
Test: atest AdminSecondaryLockScreenControllerTest
Change-Id: I6d60bc35a4e8f74b1da55b042582a2f2fa89d57f
9 files changed