Enforce one HAL per domain.

HALs are intended to be limited responsibility and thus limited
permission. In order to enforce this, place limitations on:
1. What processes may transition into a HAL - currently only init
2. What methods may be used to transition into a HAL - no using
   seclabel
3. When HALs exec - only allow exec with a domain transition.

Bug: 36376258
Test: Build aosp_marlin, aosp_bullhead, aosp_dragon. Neverallow rules
      are compile time assertions, so building is a sufficient test.

Change-Id: If4df19ced730324cf1079f7a86ceba7c71374131