Use mWindowContext instead of re-creating to avoid stale Resource

- ag/14286682 changed to re-create window context in onConfigChange, which regressed the stale Resource problem, changed back to use mWindowContext instead
- Removed unnecessary extraInternalDisplays empty check, as empty / before S is already handled inside Display Info CTOR

Test: Change display size between default and large, taskbar is enabled/disabled correctly
Bug: 181215299
Change-Id: I7397d2dc1a1ce8e132e706e9de592558df6a18e7