Small cleanups to VirtualDeviceParams recents API

This addresses some API council feedback on the previous approach, by
switching to using a new @PolicyType POLICY_TYPE_RECENTS for use with
the more generic setDevicePolicy/getDevicePolicy methods instead of
having separate methods just for recents policy.

Bug: 261697036
Test: atest VirtualDeviceManagerServiceTest CtsVirtualDevicesTestCases#VirtualDeviceParamsTest
Change-Id: I824549cf89048032045303d797637cd491c2c195