Improved WallpaperService.Engine tests

Using final variables when possible, also testing if
overlaping invalidation requests will be posted to
the handler.

Bug: 63585249
Test: runtest -x services/tests/servicestests/src/com/android/server/wallpaper/WallpaperServiceTests.java
Change-Id: I01cdbdfd5313e55d7a58c77f705aea0f6f817518