Reland^2: "init: run property service in a thread"

It's been a long standing issue that init cannot respond to property
set messages when it is running a builtin command.  This is
particularly problematic when the commands involve IPC to vold or
other daemons, as it prevents them from being able to set properties.

This change has init run property service in a thread, which
eliminates the above issue.

This change may also serve as a starting block to running property
service in an entirely different process to better isolate init from
handling property requests.

Reland: during reboot, init stops processing property_changed messages
from property service, since it will not act on these anyway.  This
had an unexpected effect of causing future property_set calls to block
indefinitely, since the buffer between init and property_service was
filling up and the send() call from property_service would then
block.  This change has init tell property_service to stop sending it
property_changed messages once reboot begins.

Test: CF boots, walleye boots, properties are set appropriately
Change-Id: I26902708e8be788caa6dbcf4b6d2968d90962785
(cherry picked from commit 1ab3dfca)