Helper class for Fragments management
your code looks very good, there are only minor issues:

1) naming

since you don't create any instances of FragmentsManager you should consider to rename it to FragmentsUtility. An Utility class provides methods to help you with your code while a Manager is an instance that does the work for you.

boolean should beginn with prefix is (or rarely as an alternative has/can/should) (in Java it's convention to use camelCase)
so rename your boolean add_to_backstack into boolean isAddedToBackstack rather (btw. in java it's convention to use camelCase)

2) comments

remove commented code!

3) javadoc

you provide some javadoc on the addFragment methode which is very crude - get things done and finish it!

the same applies to replaceFragment.