Do not advance mLastEventTime when enabling or disabling user activity.

Fixes a race condition between the keyguard and power key that sometimes resulted
in the power key failing to turn off the screen.

Fixes bug b/2212852 (Power button does not put phone to sleep)

Change-Id: I92498c5fddf2bfc32b105ea1a7ea14a06d83888e
Signed-off-by: Mike Lockwood <lockwood@android.com>