Use D8 --release for default-optimized targets

D8 uses --debug by default, whereas R8 uses --release by default.
For targets that default to R8 usage (e.g., apps), but override this
default, we still want D8 to run in release mode, preserving semantics
as much as possible between the two.

Note: This will have no effect for library targets (e.g., framework.jar)
that use D8 by default

Test: m + presubmit
Bug: 389956793
Flag: EXEMPT low-risk bugfix
Change-Id: I0bb4126685896b63052afbdf4274515e70a95c18