Fix DevicePolicyManagerTest mocking

Prefer using doReturn+when instead of when+then for spied objects that
are not pure mocks. This avoids implicit invocation of the real spied
method implementation, with potentially invalid/unsupported values, when
the method override is applied.

Bug: 418727125
Test: atest DevicePolicyManagerTest
Flag: build.RELEASE_USE_SYSTEM_FEATURE_XML_FOR_UNAVAILABLE_FEATURES
Change-Id: I3dc464e78b5c460ed036c511baa3aac8a24145b8