Fix memory leak of DataSaverPreference

When DataSaverPreference is shown repeatedly, memory leak happens
since DataSaverBackend.Listener is not removed on onDetached().

TEST: Repeat enter and exit "Data usage" for
several times and check if the count of activity
is not increasing.

Bug: 72581115
Change-Id: Ic4a72de049bb87f1e522fe46dd0fb4bb58ed65a5