Add support for custom first day of week

[sndnv]

Is your feature request related to a problem? Please describe.
I want to be able to allow users to select the first day of the week, regardless of the device's locale.

Describe the solution you'd like
MaterialDayPicker.firstDayOfWeek is already available but it is private; it should be made public and it should correctly update the picker when a new value is set.

Describe alternatives you've considered
I tried to set the value directly using reflection, however, the picker did not update as expected.

Additional context
n/a

[gantonious]

Thanks for your contribution. It should now be available in:

implementation 'ca.antonious:materialdaypicker:0.7.4'

[sndnv]

Awesome, thanks!