fix system_server dex2oat exec

Addresses the following denial:

  W/system_server( 2697): type=1400 audit(0.0:9): avc: denied { execute } for name="dex2oat" dev="mmcblk0p31" ino=118 scontext=u:r:system_server:s0 tcontext=u:object_r:dex2oat_exec:s0 tclass=file permissive=0

Bug: 16317188
Change-Id: I168842b3e281efcb0632049632ed3817c2025e4d