init: move the system property writer implementation

Move the system property writer implementation into bionic to keep
it next to the reader implementation and allow for better testing.

(cherry picked from commit 9f5af635010a7ba92edf1fca543f7271cc9d75c8)

Change-Id: Idf6100d1d0170751acd5163a22597912bff480f0

Conflicts:
	init/property_service.c