Use d8 when on eng builds

This is consistent with all of the other ways that eng builds are
faster, but do not match the expected performance of user or userdebug
builds.

We still use r8 if a module has `optimize: { enabled: true,}`.

Bug: b/374975543
Test: manual, TH
Change-Id: I28c6de4472f4df1f791758294d45b67acc0a7b7c