fix(input): don't highlight container when readonly input is focused

[crisbeto]

Prevents readonly inputs from being highlighted when they're focused. This is consistent with the native input behavior.

Note: This was introduced initially through #5776, but it looks like it didn't make it through the transition to mat-form-field.

[andrewseguin]

From internal testing - looks like this a regression of the native input's readonly attribute.

e.g. <input [readonly]=true> will result in <input ng-reflect-readonly=true> instead of <input readonly>

[angular-automatic-lock-bot]

This issue has been automatically locked due to inactivity.
Please file a new issue if you are encountering a similar or related problem.

Read more about our automatic conversation locking policy.

_{This action has been performed automatically by a bot.}