Added margins to ListView

message_item.xml has been reformatted.
Created new LinearLayout that contains a RelativeLayout which allows us
to space the RelativeLayout within the LinearLayout.

Also added new dimens to dimens.xml that include the horizontal and
vertical margins.

RelativeLayout contains these margins.

Also added paddingBottom and clipToPadding attributes to ListView in
fragment_inbox.xml that will put some padding on the ListView. The
clipToPadding being false means that the padding does not stop where an
item on the list stops.

Also added a content description for our message icon.