Make d8-on-eng a stand-alone opt-in feature

Allow a module to override optimization on eng builds only.

This adds `optimize.d8_on_eng` so that the module does not need to check
the build variant itself, but can override the calculation of
`optimize.enabled` for eng builds.

Bug: b/374975543
Test: manual, TH
(cherry picked from https://android-review.googlesource.com/q/commit:4ee862c4f17b04ee8a848f5ea8ac905be42e5209)
Merged-In: I5284072e91063520cece0050eef0a7eefff54323
Change-Id: I5284072e91063520cece0050eef0a7eefff54323