[Material] Create a FloatingActionButton ThemeData and honor it within the FloatingActionButton (#28735)

Adds a FloatingActionButtonThemeData so that FABs can be themed at the Theme level and independently. The properties that are now on the theme must be defaulted at the build level, in order to respect any contributing theme properties. Because of this, some tests had to be modified to look at properties after they are built. Also, since default behaviors are now tested in the FAB Theme test, some default tests in the FAB test no longer applied and were removed.

The themable properties are:
-backgroundColor
-foregroundColor
-elevation
-disabledElevation
-highlightElevation
-shape