MediaSession: Remove pending callback messages when setCallback(null) is called.

setCallback(null) is supposed to clear registered callbacks.
Previously, setCallback(null) doesn't remove pending callback messages
and so belated callbacks are called quite frequently after
setCallback(null) is done. This CL reduces major number of such cases by
removing pending callback messages.

Bug: 63446360
Test: Ran cts test
adb shell am instrument -w -e class \
android.media.cts.MediaSessionTest#testSetCallbackWithNull \
android.media.cts/android.support.test.runner.AndroidJUnitRunner

Change-Id: I7192b0a61a2114390f10734bbb48aeac56a19fe3