Secondary Lockscreen API: improve documentation and rename onSurfaceReady method.

- Documentation clarity and method rename per API review feedback.
- Specifying in documentation and implementation that the implementing service must be exported by the Profile Owner.

Bug: 150866056
Bug: 136085151
Test: atest FrameworksServicesTests:DevicePolicyManagerTest
Test: atest KeyguardUpdateMonitorTest
Test: atest AdminSecondaryLockScreenControllerTest
Change-Id: I58175bd6cf8936f5b1267625ca15b4f9c57f4144