Keep mCanAffectSystemUiFlags to be false while in PiP

Introduced in ag/14105379 and it appears that a Window should not be
able to affect system ui flags when it's in PiP mode. Reset this flag
when we're leaving PiP mode.

Video: http://recall/-/aaaaaabFQoRHlzixHdtY/dH4cGuTxpEYpm2myTNraJI
Bug: 234082669
Test: follow the reproduce steps in bug, see also the video \
      1. status bar does not flash when launch another app \
      2. status bar always visible when launch Settings \
      3. status bar turns back invisible when exiting to fullscreen
Change-Id: I4d64c19750e69a33ad9ec5962ee4e62320b921af