Fix warnings surfaced by error-prone.

Bug 36033775

Error-prone discovered that mAdded is created after usage, but
can be accessed on any thread. This makes mAdded a final value
so that it cannot be created on one thread and accessed on another.

Test: ran cts fragment tests
Change-Id: I4e655a6cef9d924f392c021d9d2a9ae2d0dc0519