Remove neverallow preventing hwservice access for apps.

Same-process HALs are forbidden except for very specific HALs that have
been provided and whitelisted by AOSP.  As a result, a vendor extension
HAL may have a need to be accessed by untrusted_app.  This is still
discouraged, and the existing AOSP hwservices are still forbidden, but
remove the blanket prohibition.  Also indicate that this is temporary,
and that partners should expect to get exceptions to the rule into AOSP
in the future.

Bug: 62806062
Test: neverallow-only change builds.  Verify new attribute is in policy.
Change-Id: I6d3e659147d509a3503c2c9e0b6bb9016cc75832