Implement setPolicy in DPMS

Implement a first version of setPolicy in DPMS. It can now handle the
scope parameter and applies the default value.

A more generic implementation will be added in later CLs, currently only
supports SCREEN_CAPTURE_DISABLED.

Bug: 434655549
Fixed: 434615264
Flag: android.app.admin.flags.policy_streamlining
Test: CTS
Change-Id: I75d23509030bad725865843503607a97c6e53630