Avoid two SELinux related error messages at boot time

It is expected that /metadata/ota/rollback-indicator and /metadata/gsi
don't always exist, so don't call selinux_android_restorecon() on them
when they don't exist.  This eliminates the following error messages:

0     0 E selinux : SELinux: Could not get canonical path for /metadata/ota/rollback-indicator restorecon: No such file or directory.
0     0 E selinux : SELinux:  Could not stat /metadata/gsi: No such file or directory.

Test: Booted Cuttlefish and verified the error messages are gone
Change-Id: I94c998556c85adde5f11f134178219ba7880c2be