respect $OUT_DIR_COMMON_BASE with squisher

If $OUT_DIR_COMMON_BASE is set, signapk.jar was
being called directly from $ANDROID_BUILD_TOP/out
and caused the build to fail.
Change-Id: I9f8bfb22140866bd2c91a675efa954433431220c
Signed-off-by: Roman Birg <romanbirg@gmail.com>