Refactor DisplayWindowSettingsProviderTests.

This commit refactors the DisplayWindowSettingsProviderTests to reduce
boilerplate code and improve readability. A shared
DisplayWindowSettingsProvider instance is now initialized in the setUp
and reused across multiple tests, avoiding repeated instantiation.

Additionally, display information such as DisplayInfo objects and unique
identifiers are cached as member variables. Helper methods like
readDisplayWindowSettingsFromStorage and assertExpectedSettings have
been introduced to encapsulate common setup and assertion logic, making
the individual tests cleaner and more focused.

Bug: 387193964
Flag: EXEMPT refactor
Test: atest WmTests:DisplayWindowSettingsProviderTests
Change-Id: I4d84928fae57e90a085ef1fb86d0348c48ad1f65