Allow untrusted apps to execute binaries from their sandbox directories.

Various third party apps come with their own binaries that they write out to
their sandbox directories and then execute, e.g.:
audit(1386527439.462:190): avc:  denied  { execute_no_trans } for  pid=1550 comm="Thread-79" path="/data/data/com.cisco.anyconnect.vpn.android.avf/app_bin/busybox" dev="mmcblk0p23" ino=602891 scontext=u:r:untrusted_app:s0:c39,c256 tcontext=u:object_r:app_data_file:s0:c39,c256 tclass=file

While this is not ideal from a security POV, it seems necessary to support for
compatibility with Android today.

Split out the execute-related permissions to a separate allow rule as it
only makes sense for regular files (class file) not other kinds of files
(e.g. fifos, sockets, symlinks), and use the rx_file_perms macro.

Move the rule to untrusted_app only so that we do not permit system apps
to execute files written by untrusted apps.

Change-Id: Ic9bfe80e9b14f2c0be14295c70f23f09691ae66c
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>