Vertically stack AlertDialog buttons when necessary (1/2)

This CL makes the Compose AlertDialog behave the same as our View
SystemUIDialog when there is not enough space to stack the buttons
horizontally. When this happens, we now stack them vertically.

See b/283817398#comment9 for a video.

Bug: 283817398
Test: atest ComposeDialogScreenshotTest
Change-Id: I8b6320b124608b2864d990f2683abe766232fd29