Add dialog test util to ViewScreenshotTestRule

This CL adds a dialogScreenshotTest {} util function to
ViewScreenshotTestRule so that we can easily add screenshot tests for
any kind of dialog. See ag/19066186 for an example.

Bug: 230832101
Test: atest SystemUIGoogleScreenshotTests
Change-Id: If078c040809310c5c9a0ab24293d9131037fc42a