Address API review

* Rename POLICY_SCOPE_PARENT to POLICY_SCOPE_PARENT_USER.
* Clarify what happens when setPolicy() is set to null.

Fixes: 435416374
Bug: 434919316
Flag: android.app.admin.flags.policy_streamlining
Test: None
Change-Id: I829dee985c46d0818e2acef5a0664cd0afa2b468