Throttle proximity sensor values so we do not process more than one change a second.

This is to prevent a noisy proximity sensor from causing the screen to turn on and off
in rapid succession, which can trigger race conditions in the keyguard manager.

Change-Id: I2c6e54e4b41716e038854ed9a827c901579c8865
Signed-off-by: Mike Lockwood <lockwood@android.com>