Give healthd permission to write sys properties to allow quick boot from charger mode.

Property being set: sys.boot_from_charger_mode. If healthd attempts to write
this property without the policy changes we get the following audit message:

[   45.751195] type=1400 audit(1403556447.444:7): avc:  denied  { write } for pid=99 comm="charger" name="property_service" dev="tmpfs" ino=3229 scontext=u:r:healthd:s0 tcontext=u:object_r:property_socket:s0 tclass=sock_file permissive=0

These changes are needed to support the following system/core commit:
faster booting from charger mode
* Ieec4494d929e92806e039f834d78b9002afd15c4

Change-Id: I9f198cd73c7b2f1e372c3793dc2b8d5ef26b3a0f