Do not dismiss session ui if the close dialog reason is assist

When assistant is launched from the hardware button, it will try to
close system dialog with assist reason and launch a new UI again.
From the current logic, the voice system service doesn't filter
the assist reason to hide itself. If the session already exists,
it will hide itself and get another onShow immediately. This
causes unnecessary ui animations for hide/show from the user
perspective. To fix it, this change will filter the assist reason
to hide the session.

Bug: 257275314
Test: atest CtsVoiceInteractionTestCases
Test: use hotware button to trigger assistant and monitor the
animation status

Change-Id: Ibe636ad0b6776a1dbdf7f709a9ef5e804c6b015a