MonthByWeekFragment: Don't init loader in certain cases

The new androidx LoaderManager requires to be NonNull. Strip out the cases
that currently returns null and add them as check before initializing the
LoaderManager.

Also switch to androidx methods to get the LoaderManager instance.

Change-Id: I4ea2bdb5bf196af2add4430360ec265184de0a32
Signed-off-by: Aayush Gupta <aayushgupta219@gmail.com>