Add method to check for ongoing video call or conference call.

Replace the isVideoCallPresent method on Phone (which is used only in the
accessibility settings screen) with a version that determines if there are
any ongoing video calls, downgraded video calls, or conference calls.

Test: Manual testing.
Bug: 28512893
Change-Id: If8b5db80aa0147234f6c4c0a8adf85f4643a2680