1-arg variant of hal_impl_domain

This improves readability and consistency for HAL implementation
domains which have only one implementation.

Test: No change to policy according to sesearch
Test: No change to which types are associated with haldomain according to "sepolicy-analyze <sepolicy file> attribute haldomain"
Bug: 34180936
Change-Id: Ice599ea4971cdfbd8b835b1fd02ad1e14c7a0386