power: allow easy access to number of powerhal hints

Add a POWER_HINT_COUNT to power hints.

Bug 1183244

Reviewed-on: http://git-master/r/173711
(cherry picked from commit 7034546f030ffdb3f588a861843cce030d8b8bdc)

Change-Id: Iab10da1e043737f9e0a2c837f985f6cb5e11811a
Reviewed-on: http://git-master/r/326340
Tested-by: Jean Huang <jeanh@nvidia.com>
Reviewed-by: Peter Zu <pzu@nvidia.com>
Reviewed-by: Jean Huang <jeanh@nvidia.com>