Remove static Environment dependency from DPE

DPE was reading the policy folder from the static Environment.

There already is class that wraps the logic for folders, use that one
instead. Following DRY and making dependencies visible like this makes
the code a bit easier to test and change.

Test: atest FrameworksServicesTests_devicepolicy
Test: atest CtsDevicePolicyTestCases
Bug: 416180519
Flag: EXEMPT refactoring
Change-Id: If194581f1b71716b2e7b86c28c2594848c2c804b