audiomanager: fix volume group callback

The AudioVolumeGroup change callback for a given group may be lost.
It is due to a poor copy paste in the Handler implementation where
the previous messages where removed.
In the case of volume group, it is forbidden to remove handled message
otherwise previous group change will not be reported.

Test: run cts-dev -m CtsMediaTestCase --test android.media.cts.AudioManagerTest#testVolumeGroupCallback
Bug: 136121584
Change-Id: I7434e66f02143c7da0f7156347d899e81467ee98
Signed-off-by: François Gaffie <francois.gaffie@renault.com>