Add 'ro.cpuset.default' system property for default cpusets.

Normally 'writepid' is used to add a process to a particular cpuset. However
certain systems with big/small cores might need to specify a default cpuset for
system processes which do not explicitly specify one. Add an option to use
'ro.cpuset.default' system property to specify default cpuset for system processes
which do not explicitly write to /dev/cpuset/... with 'writepid' option.

The cpuset name specified in ro.cpuset.default is just the cpuset name, e.g.
'/system-background', '/foreground', or simply '/' for the "root" cpuset.

Bug: 28550814
Test: `m -j32` succeeds for aosp_sailfish-eng. Phone boots successfully.
      Also tested manually with debug trace messages on emulator with different
      combinations of values for 'ro.cpuset.default'.
Change-Id: I501727fa5ee3f4bb7a938fa104b81a404b616633