Clean the flag enable_connected_display_management

Remove all code using it, and the flag itself.

Updated the tests to remove those assuming the flag is off and adapt the
other ones.

Test: atest DisplayManagerServiceTest
Test: atest LogicalDisplayMapperTest
Test: atest ExternalDisplayPolicyTest
Flag: EXEMPT (flag cleanup)
Fix: 388021153
Change-Id: I4d53806a66a2e85f49eb6db57c49fe6c2cfb869e