Fix: Flattend APEX is breaking build

When TARGET_FLATTEN_APEX=true, individual files in an APEX are exported
to make. However, because they lack LOCAL_SOONG_CLASSES_JAR,
soong_java_prebuilt.mk get confused and the build breaks.

Fixing the bug by correctly emitting LOCAL_SOONG_CLASSES_JAR and
LOCAL_SOONG_HEADER_JAR.

Test: TARGET_FLATTEN_APEX=true m
out/target/common/obj/JAVA_LIBRARIES/com.android.conscrypt.conscrypt_intermediates/classes.jar

Change-Id: I65ce86ec825f978a43715146bbd5c1d5af513de0