Fix: APEX gets .jar.jar suffix when TARGET_FLATTEN_APEX is true

soong_java_prebuilt.mk sets LOCAL_MODULE_SUFFIX := .jar. Therefore, we
need to remove .jar suffix from LOCAL_MODULE_STEM, otherwise, we
will have .jar.jar suffix.

Bug: 123092860
Test: TARGET_FLATTEN_APEX=true apex.test
Check /system/apex/apex.test/javalib/core-oj.jar is there.

Change-Id: Ie19b20523753ec32ab16266f63d26c4df685cc34