When building unbundled apps, link to the prebuilt stubs lib

There was a bug that when an unbundled app is built without SDK
(LOCAL_SDK_VERSION isn't set), it is linked to the nonexisting stubs
library from java_sdk_library. The built stubs libs are not created for
unbundled builds. For unbundled builds, we need to use the prebuilt
stubs lib.

Bug: 77575606
Test: m -j
Test: make -j dist
ANDROID_BUILDSPEC=vendor/google/build/app_build_spec.mk

Change-Id: I70cced1c872053e151fea157e3a6d218b1fab9dc