init: avoid ERROR log due to missing SEPolicy.zip

One of the first ERROR messages in logcat of a normal boot of Cuttlefish
is from failure to open SEPolicy.zip.  This condition is expected.
Therefore don't try to load SEPolicy.zip when it doesn't exist.  This
replaces the following log messages:

0     0 I init    : Error: Apex SEPolicy failed signature check
0     0 I init    : Loading APEX Sepolicy from /system/etc/selinux/apex/SEPolicy.zip
0     0 E init    : Failed to open package /system/etc/selinux/apex/SEPolicy.zip: No such file or directory

... with just:

0     0 I init    : No APEX Sepolicy found

Change-Id: If3a77407c35130165df5782b9ef91912e8374dbf