init: simplify async restorecon

In the future, property service may run in its own thread or process,
which means that PropertyChildReap() needs to be refactored to not run
as part of the init signal handler.

The new method spawns a new thread that handles the queue of paths
that require restorecon. It then communicates back to property service
via android::base::SetProperty(). Property service distinguishes the
thread from other callers of SetProperty() by checking the pid in the
credentials for the socket connection, thus avoiding dependencies on
the rest of init.

The new method also drops the genericness, since restorecon is the
only function that we should ever need to run asynchronously

Test: async restorecon works, including with queued requests
Change-Id: I2ca00459969e77b1820776dac23d0a0d974e330b
(cherry picked from commit fe815417)