Enable per-policy rewards.

Reserve 0 for the base policy event IDs so that individual policies can
create their own rewards without interference or confusion.

Bug: 243987091
Test: atest frameworks/base/services/tests/mockingservicestests/src/com/android/server/tare
Test: atest frameworks/base/services/tests/servicestests/src/com/android/server/tare
Change-Id: I96e092534996c27dc45981301de6bc368798f32a