/dev/hw_random should be readable by system_server for EntropyMixer to
be able to use it.

Bug: http://b/179086242
Test: adb shell logcat | grep Entropy
Test: android.permission.cts.FileSystemPermissionTest#testDevHwRandomLockedDown
Change-Id: I7dd7faf300f5ce3cab8d02c69be7915238cf61da