Preserve debug info for unoptimized services.jar

A recent change enabled very basic stripping of services.jar for the
unoptimized path where OPTIMIZE_SYSTEM_JAVA=false, avoiding some
unnecessary bloat from permissions deps. However, by enabling R8
for this path, it implicitly enabled compilation with `--release`,
which strips debug info.

Explicitly set `--debug` for this scenario to ensure debug info is kept.
Also update the associated comments to reflect more recent changes to
this condition.

Test: OPTIMIZE_SYSTEM_JAVA=false m services
Bug: 298311479
Change-Id: If04075c9bb7d4200b26ec8dc622f42500a312f80