bug(datepicker): Comparison dates are relying on single sensory characteristic

#### Reproduction

Steps to reproduce:
1. Go to the [datepicker examples](https://material.angular.io/components/datepicker/examples)
2. Go to the "Date range picker comparison ranges" example
3. Open the calendar dialog


#### Expected Behavior

it should rely on atleast two sensory characteristics to indicate the comparison dates, say, consider adding tooltip over the comparision dates as "Test comparison dates"

#### Actual Behavior

The comparison dates are relying on single sensory characteristic color

##### Additional Information
This is copied from an [internal bug report](http://b/195600607) found during a11y testing.

Start with the Date range picker comparison ranges example in the datepicker examples and reproduce the issue by opening its calendar dialog. Trace how comparison dates are presented, then verify that they are conveyed through more than color, such as an additional accessible indication or tooltip, and confirm the result in the example.

Written by the indexing model from the issue text.