Revert "Revert "Use the platform SDK version from the config.""

This reverts commit 74b8b55b.

Test: OUT_DIR=ndk-out DIST_DIR=ndk-dist \
          build/soong/scripts/build-ndk-prebuilts.sh
Bug: None

Change-Id: Ia320609ec6fb382b13ff9007a6496b73198ff871