In general, apps shouldn't be executing data from their writable data
directories. Allowing this is a security risk and use cases for this are
almost always anti-patterns where saner alternatives are available such
as using one of the standard systems for shipping libraries (extracted
by the package manager or aligned/uncompressed in the apk) or using the
existing package system to handle plugins. It's reasonable for the
untrusted_app domain to have this (not just for backwards compatibility)
for priv_app should be held to a higher standard.

Ideally, untrusted apps would be able to opt-in to disabling this and
then the default could then be switched at a new API level. It could do
more than just hardening apps not requiring it by having documentation
explain the risks and offer alternatives to reduce 'legitimate' use. The
base system could disable it for all of the bundled untrusted apps.

Change-Id: I4efcfaf01c6b6c33c39e98c22a1934e8892e2147