Remove unused ScreenshotData.contextUrl field.

Assist content is hadled outside of screenshot data.

Also a minor tweak to make the kotlin more idiomatic and use real int
constants in tests (resolving an IDE warning).

Bug: NONE code cleanups.
Flag: EXEMPT code cleanups.
Test: atest com.android.systemui.screenshot
Change-Id: I13aa753b8821e56d54f3c56b59b62b6487c55d20