Fix AudioManagerTest.testVolume()

This test was broken by commit 45c90cef: now VolumePanel forces the stream type
controlled by adjustSuggestedStreamVolume() even from other processes.

The test should not specify FLAG_SHOW_UI so that VolumePanel is not displayed.

Issue 6418771.

Change-Id: Ib6eb53e6e3262fbbe301ca485a1018c80200e0fd