Introduce TARGET_BUILD_UNBUNDLED_IMAGE

- TARGET_BUILD_UNBUNDLED_IMAGE is similar to TARGET_BUILD_APPS, but
its targets are the unbundled partitions instead of apps.
- Rename TARGET_BUILD_APPS_USE_PREBUILT_SDK to TARGET_BUILD_USE_PREBUILT_SDKS
because it is used even without TARGET_BUILD_APPS.
-Instead of TARGET_BUILD_APPS, use TARGET_BUILD_USE_PREBUILT_SDKS
to build java modules with prebuilt sdks, and propagate to Soong.

Bug: 160390776
Test: TARGET_BUILD_UNBUNDLED_IMAGE=true m vendorimage
Change-Id: Ie096212ccbcca0018baae55e106af693b002c9e5