[DatePicker] Support Custom date input hint

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**Is your feature request related to a problem? Please describe.**
Currently I believe we can't set the hint as we want. We only able to change it by overriding the word using these strings res
```
y
m
d
```
but we need to follow the defined format dd/mm/yy.

**Describe the solution you'd like**
It would be nice to have if we can set the custom hint as we want maybe from dd/mm/yy to Day/Month/Year

**Describe alternatives you've considered**
Set the string res that won't printed as double word i.e I put d it will become dd, if I want to put Day it will printed as DayDay.

Start by locating the Android DatePicker text-input implementation and the existing mtrl_picker_text_input_year_abbr, month_abbr, and day_abbr string resources. Check how those resources produce repeated hints, then define the expected behavior for custom formats such as dd/mm/yy and Day/Month/Year; done means the requested hint can be customized without duplicated words.

Written by the indexing model from the issue text.