Splits MenuDropdown styles into modern and legacy styles.
Leaves a common MenuDropdownCheckbox.less file (for lack
of a better name) for common styles related to the checkbox
behaviour and dropdown menu resets.
Adds a new file in common/mixins.less for a shared
dropdown mixin.
Bug: T312157
Change-Id: Ib60b25ea3121843ed5ed54820d6295a81289b332
