Provide platform build flag to R8/D8

Provide the `--android-platform-build` flag to R8/D8 for targets that do
not specify a stable min SDK. This includes all targets that do not
specificy an SDK version (e.g., services.jar, SystemUI.apk) and all
targets that specify core_platform (e.g., framework.jar).

Follow-up work will expand the set of cases that qualify for this flag,
but for now this captures some of the highest priority targets.

Bug: 232073181
Test: m + presubmit + validate verbose.log
Change-Id: Idad32c80490a101c049b7f937807a615564ca8ef