Fix snooze/bubble button padding

* Reduce the conversation expander width to 48dp.
  This matches the expander width for normal notifications.
* Remove end padding for snooze or bubble buttons.
* Use layout to ensure 12dp end padding for actions iff
  neither of the buttons is present.

Bug: 163626038
Test: manual
Change-Id: Ic780f98ad74a2e148db2716aca52abe5137fa4a1