[DatePicker] Support Custom date input hint

[singgihsaputro]

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

<string name="mtrl_picker_text_input_year_abbr" tools:override="true">y</string>
<string name="mtrl_picker_text_input_month_abbr" tools:override="true">m</string>
<string name="mtrl_picker_text_input_day_abbr" tools:override="true">d</string>

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.