Remove platform_app from neverallow execute from /data

Apparently some manufacturers sign APKs with the platform key
which use renderscript. Renderscript works by compiling the
.so file, and placing it in the app's home directory, where the
app loads the content.

Drop platform_app from the neverallow restriction to allow partners
to add rules allowing /data execute for this class of apps.

We should revisit this in the future after we have a better
solution for apps which use renderscript.

Bug: 29857189
Change-Id: I058a802ad5eb2a67e657b6d759a3ef4e21cbb8cc