app_process: change app_process a symlink of primary version

Change app_process to be a symlink to the primary version
and have app_process32/app_process64 be the 32-bit/64-bit
versions respectively.

This will make it possible to use the primary zygote for
am/pm commands and other scripts that use app_process and
don't need a specific zygote.

Signed-off-by: Yongqin Liu <yongqin.liu@linaro.org>
Change-Id: Ibef545994eb95ddc82101c95df613ad1d06345e6