Introduce enum policy definitions

The policy processor can now extract enum options and puts them in
`policies.json`.

Bug: 440992241
Test: atest frameworks/base/tools/processors/devicepolicy/test/src/android/processor/devicepolicy/test/PolicyProcessorTest.kt
Flag: android.app.admin.flags.policy_streamlining
Change-Id: I25b847ab0ff392ca22269ca7999c3ee650203da4