init: use a property instead of file to communicate cold boot done

Ueventd can't set properties currently, but this is an artificial
limitation, since ueventd communicates to init that it has finished
cold boot via a file, and init polls this file instead of returning to
the epoll loop, where properties are handled.

This change replaces that file with a property and thus frees ueventd
to be able to set properties.

Bug: 62301678
Test: boot, check that properties are set
Change-Id: I985688e9299456efcb2dfeef9b92668991aa9c05