DatePicker A11Y fix time announced for date widgets

* introduced a new method (getFormattedCurrentDate) which formats
the date for all DatePickerDelegates
* implemented (moved) onPopulateAccessibilityEvent in the
AbstractDatePickerDelegate thus all announcements for date pickers
use the same format
* note: added FORMAT_SHOW_WEEKDAY to the current format because
I find the information valuable

E.g:
* Before: "Thu Jun 16 11:49:14 GMT+01:00 2016"
* Now: "Thursday, 16 June 2016"

Bug: 29099922
Change-Id: I94447067f10659747ae310f9e07cf3f487c97e05