RELEASE_ACONFIG_VALUE_SETS is a reserved flag

Disallow setting the flag via protobuf files, and adjust it internally
to be more like any other build flag.  This makes the generated output
more consistent.

Also default to TARGET_RELEASE if no release is given to `build-flag`.

Bug: 328495189
Test: manual
Change-Id: I9db57137fc4e5ed42a38adc939c430826afe4f63