TaskOverlayFactoryGo: Keep persistent instance of AssistContentRequester

Holding a reference to AssistContentRequester prevents it from being deleted by the garbage collector while it is still in use.

Bug: 189985872
Test: Manual (Pixel 3A)
Test: m -j RunLauncherGoGoogleRoboTests
Change-Id: I204aab907175a171c496c3bd146a8b23d2bbcf16