Add CPU boosting hooks

 * Boost CPU to max during scrolling
 * Boost CPU to max on activity start/resume
 * Inspired by a patch from CodeAuora.

Change-Id: I72f2c2851b01fbceabddead03a4c3a7a0526ca61
power: Add CPU boosting interface

 * Boosts CPU using Power HAL for the given duration.
 * Duration is given in microseconds.
 * Power HAL must implement POWER_HINT_CPU_BOOST.

Change-Id: Ic79baf7e3d0f75483c2fe8a242b4c3d93368b68b