Fix HearingDevicesDialogDelegateTest failed

The HearingDevicesDialogDelegateTest is failing with a NullPointerException if it runs after the FontScalingDialogDelegateTest. This issue seems related to the changes introduced in ag/31234175.

Dismiss the dialog in FonScalingDialogDelegateTest#changeFontSize_buttonIsDisabledBeforeFontSizeChangeFinishes first to prevent the test failing.

Flag: EXEMPT bugfix
Bug: 390185052
Test: atest SystemUITests:com.android.systemui.accessibility
Change-Id: I38c585fcb140ab6e3e52d70de210c8e338e6228f