Fix DPMS unit tests.

Currently these tests fail because of an attempt to register the same
service more than once in a static map, which leads to an exception.
Make DevicePolicyManagerTestable unregister them before constructing
DPMS, so that it doesn't have to be done in every test.

Bug: 228842216
Test: atest com.android.server.devicepolicy
Change-Id: I43d10193779edf7909d77863f13ecdccfa383473