Always turn on TARGET_BUILD_USE_PREBUILT_SDKS for unbundled build.

Before this change, TARGET_BUILD_USE_PREBUILT_SDKS was turned on only if
TARGET_BUILD_APPS or TARGET_BUILD_UNBUNDLED_IMAGE is set. After this
change, TARGET_BUILD_USE_PREBUILT_SDKS is always turned on if
TARGET_BUILD_UNBUNDLED is set.

We need this change because we need to build ART services on the thin
manifest, and ART's build script only sets TARGET_BUILD_UNBUNDLED.

Bug: 177273468
Test: (on aosp-master-art) . ./build/envsetup.sh && lunch arm_krait-eng && art/tools/buildbot-build.sh --target
Change-Id: I1b5f63043d518ac8306bd172ef8afd69ab3ae8e5