Fix material style errors in suggestion popup window.

This CL addresses the material style errors from UX team.

- Corners should have a radius of 2dp.
To use the same way with floating toolbar, make the popup window transparent and
set floatingToolbarPopupBackgroundDrawable as the layout background. By this
change, TextSuggestionsPopupWindow style is no longer necessary, removed it.

- Background color for both sections should be #FFFFFF.
It turned out that the window default background is not white. By addressing
above item, this issue is fixed too.

- Divider line alpha=.12
This is fixed by using attr/listDivider instead of attr/dividerHorizontal.

- Remove 8dp padding below last suggestion
Removed 8dip paddingBottom from ListView.

Bug: 15347319

Change-Id: I579c0cc5b7f0dd337bff54af77828b8af25b13d2