Fix NPE in Desktop tests when secondary display bug fix flag is on.

This pulls a context from the DisplayController, which needs to be
explicitly set as returned given that it's a mock in tests.

Test: atest
Bug: 382023296
Flag: com.android.window.flags.enable_bug_fixes_for_secondary_display
Change-Id: I55e3267dc4cdbaa9f45474c866e2d8250e3cb896