Fix DisplayWindowSettingsTest to restore overscan settings

This CL restores overscan settings to default values after finishing tests
that modify overscan settings of primary display. This CL also fixes
DisplayWindowSettings#applySettingsToDisplayLocked(DisplayContent) to be
able to apply default settings value.

With this CL, we can reuse WindowManagerService instance among all
WindowTestsBase tests.

Fixes: 121296525
Test: Pass all presubmit tests in WmTests.
  $ atest --test-mapping frameworks/base/services/core/java/com/android/server/wm

Change-Id: I2f21490b3361b466c090434b7ba9402eaa82c586