sched: Provide tunable to switch between PELT and window-based stats

Provide a runtime tunable to switch between using PELT-based load
stats and window-based load stats. This will be needed for runtime
analysis of the two load tracking schemes.

Change-Id: I018f6a90b49844bf2c4e5666912621d87acc7217
Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>