Make SCREEN_CAPTURE an enum policy.

Deprecate the boolean SCREEN_CAPTURE_DISABLED and replace it with an
enum SCREEN_CAPTURE policy. Both control the same behavior, but the new
policy matches how we want policies to look going forward.

Bug: 440992241
Test: atest cts/tests/devicepolicy/src/android/devicepolicy/cts/ScreenCaptureDisabledTest.java
Flag: android.app.admin.flags.policy_streamlining
Change-Id: I8d60723d6b3758c2cc5a8f9c2f1b71636a9aeaa1