Add FOR_TESTING UserInfo Flag.

This is used to allow the setting of DeviceOwner and Device Policy
Management Role Holder during tests.

The purpose is to ensure that we don't allow setting of Device Owner on
any devices which have user data.

Test: atest CtsDevicePolicyTestCases
Bug: 256655297
Change-Id: I2f792076d6250d94b688eef361d72196f7115965