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.

(cherry picked from commit c55cf17a)

Bug: 29857189
Change-Id: I058a802ad5eb2a67e657b6d759a3ef4e21cbb8cc

app.te

@@ -442,7 +442,6 @@ neverallow {
   bluetooth
   isolated_app
   nfc
-  platform_app
   radio
   shared_relro
   system_app