init.rc: Mount /mnt with nodev,nosuid,noexec

This change adds some additional flags to /mnt. This is to reduce
the number of mounts with unnecessary flags.

Bug: 73255020
Test: aosp_sailfish still boots
Test: CtsAppSecurityHostTestCases {StorageHostTest,ExternalStorageHostTest}
Test: CtsOsTestCases StorageManagerTest
Test: find /mnt  | egrep -v '^/mnt/runtime/(default|read|write)/emulated' | \
      xargs ls -lZd  # Shows no character devices or executable files
Change-Id: I54739133119d9626ebeb2ef9a1c127f7a90fa098