Removed cpusets/schedboost build time dependency.

These two Soong product_variables were usually controlled by
ENABLE_CPUSETS and ENABLE_SCHEDBOOST in BoardConfig.mk. This change
turns the build time variables into runtime decision by checking if
a special file, "tasks", can be found under the mount points,
/dev/cpuset and /dev/stune, for cpusets and schedboost, respectively.

That special file only exists when the corresponding Linux kernel
configurations, CONFIG_CPUSETS and CONFIG_SCHEDTUNE, are set.

Bug: 34726944

Test: Tested on Sailfish with following configurations
          neither cpuset nor schedtune enabled in the kernel
          cpuset enabled, but schedtune disabled in the kernel
          both cpuset and schedtune enabled in the kernel

Change-Id: I6695b8b32b2fecb4fd995018418bbe2edc5d590f