Fix the bug in replacing a fragment with itself.

Summary:

We are facing a bug that some fragments in our app got removed
randomly. We debugged the app and it seems there is a bug in
the fragment API.

When we we are trying to replace a fragment with itself. We will
try to make the operation a null op, then both `op.fragment` and
`f` will be reset to null.

  op.fragment = f = null;

But since it's inside a loop, if the list `mAdded` still have
items to be visited, the code will continue to the next
iteration. Then it will compare the fragment with null, and
reach the line:

  mManager.removeFragment(old, mTransition, mTransitionStyle);

Eventually it will delete all left fragments in the list `mAdded`.

This diff adds a break to abort the searching after we null
the operation.

Test Plan:

1) Tested with our app. No fragment is randomly removed.

Revert Plan:

Change-Id: I5dc2c07a1bb1d1da1b0fd4e5610d970d55a81e86
Tags:
Signed-off-by: Kang Zhang <kangzhang.cn@gmail.com>