Abolish calls to shell in vold

Never use popen, just execvp directly

Test: Two tests
- Ensure Marlin device boots and vold_prepare_subdirs is called
successfully
- Try adb shell sm set-virtual-disk true, see that eg sgdisk output is
logged.
Bug: 26735063
Bug: 113796163

Change-Id: Icb34140429db85098a0118a2b833772e3620e7ac
diff --git a/private/vold.te b/private/vold.te
index a6d1001..dea24a5 100644
--- a/private/vold.te
+++ b/private/vold.te
@@ -8,8 +8,8 @@
 
 # For a handful of probing tools, we choose an even more restrictive
 # domain when working with untrusted block devices
-domain_trans(vold, shell_exec, blkid);
-domain_trans(vold, shell_exec, blkid_untrusted);
+domain_trans(vold, blkid_exec, blkid);
+domain_trans(vold, blkid_exec, blkid_untrusted);
 domain_trans(vold, fsck_exec, fsck);
 domain_trans(vold, fsck_exec, fsck_untrusted);