Don't start logcat from vsock_logcat to keep selinux happy

Start it in its own process and make it write to a named pipe created
by vsock_logcat, which in turn forwards the logs to the host side
process. As with qemu, logcat is started when vsock_logcat sets the
vendor.ser.cf-logcat property.

Bug: 129163131
Test: run locally under crosvm, check there are no avc: denied
messages.

Change-Id: I8a9b04c33f21272698e40f88e5264c1355552955