Fix bootanimation destpath if building off-tree

If the build is invoked off-tree, like:

    . build/envsetup.sh
    export OUT_DIR=/some/other/dir/out
    export DIST_DIR=/some/other/dir/dist
    breakfast chagalllte
    mka \
        target-files-package dist

the bootanimation.zip is correctly generated in $OUT_DIR.

However, if the user overrides directories after breakfast, i.e:

    . build/envsetup.sh
    breakfast chagalllte
    mka \
        OUT_DIR=/some/other/dir/out \
        DIST_DIR=/some/other/dir/dist \
        target-files-package dist

the destination path of intermediate bootanimation.zip is still
controlled by $ANDROID_PRODUCT_OUT which is set by build/envsetup.sh.
This leads to a copy error and a failed build.

The fix overrides $ANDROID_PRODUCT_OUT by the always-known PRODUCT_OUT
(make variable defined in build/core/envsetup.mk), making it error-prone
to environment variable declaration.

Also, the "sh" invocation is replaced with generic make-variable
$(shell).

Change-Id: Ic94d1b538fc01946b628f9b5d776548a03acde97
Signed-off-by: Vasyl Gello <vasek.gello@gmail.com>