[Dialog] Remove 48dp minimum height requirement from SysUI dialogs.

Requiring the dialog content area to have a minimum height of 48dp
causes there to be too much space between the dialog content and the
dialog buttons if the content is a short line of text.

This CL removes that requirement. There will always be 32dp of padding
above the dialog buttons, so there will still be sufficient space
between dialog content and dialog buttons.

Fixes: 349834936
Flag: NONE minor UI change
Test: With com.android.systemui.status_bar_screen_sharing_chips enabled,
start a screen recording then tap status bar chip -> verify there's not
too much spacing between dialog message and buttons
Test: smoke test of other SysUI dialogs

Change-Id: Ied6a8429dbf5c0ab9daeb67efab2017a55bd541a