grant installd rx perms on toybox

Installd uses cp when relocating apps to sdcard.

avc: denied { execute } for name="toybox" dev="mmcblk0p10" ino=315 scontext=u:r:installd:s0 tcontext=u:object_r:toolbox_exec:s0 tclass=file

Bug: 24617685
Change-Id: Id1a3039bbfc187c074aa50d426278964c40e4bde

installd.te

@@ -95,3 +95,6 @@ allow installd { system_app_data_file bluetooth_data_file nfc_data_file radio_da
 
 # Create and use pty created by android_fork_execvp().
 allow installd devpts:chr_file rw_file_perms;
+
+# execute toybox for app relocation
+allow installd toolbox_exec:file rx_file_perms;