Address new SELinux denial.

system/core commit 75b287b771b302c99797d812122b72f83d2f56f9 fixed
/sys/devices/system/cpu/cpu*/cpufreq/scaling_* so that it was properly
owned by uid/gid system. That started generating the following SELinux
denial:

<4>[  380.676844] type=1400 audit(1398985976.921:19): avc:  denied  { dac_override } for  pid=2033 comm="mpdecision" capability=1  scontext=u:r:mpdecision:s0 tcontext=u:r:mpdecision:s0 tclass=capability

Add mpdecision to the system group. This stops the DAC override denial
by giving mpdecision DAC read/write access.

Change-Id: I3173c95a7fc618a0cc7894cf1edac22a6c04a83e