DO NOT MERGE - Replace "lower power mode" experiment

This replaces the previous low-power mode experiment, which
discarded refresh events, with a new experiment that alters
the refresh period.

(see also I2849e5ea335c0d2509fea1c315392bce7f20451d )

The feature is enabled by specifying a nonzero value for the
"refresh skip count", which indicates the number of periods
to skip.  For example, the command:

  adb shell service call SurfaceFlinger 1016 i32 1

sets a skip count of '1', yielding a 30Hz refresh rate on a device
with a 60Hz display.  Changing the last value to '2' would set the
refresh to 20Hz.  '0' returns to the default behavior.

Bug 15523257

(cherry-pick from master I00039c22a55750e74035644c63800e4bee1c774a)

Change-Id: I9ef5539fa7da953dd97f88e7fa39be0dc20b6889