Add implementation for ALLOW_LOCK_WHILE_SCREEN_ON flag to PhoneWindowManager

This allows the lockscreen to activate after the normal screen timeout while the
screen is on.  Needed for dock apps, screen savers, etc.

Change-Id: I25c3579e4363c2b19d503fdbd45d2ff41cdd11bb
Signed-off-by: Mike Lockwood <lockwood@android.com>