Fix NPE issue when tapping speaker button on VT call ended screen

Null pointer exception occurred when tapping speaker button on
VT call ended screen. At the ending of VT call, the call is NULL and
issue caused by accessing the call.

To resolve this issue, disable speaker button on VT call ended screen
by updating button UI when button controller is disabled.

Bug: 109765937
Test: manual - Check that speaker button is disabled on the
VT call ended screen.

This is an upstream change:
https://android-review.googlesource.com/c/platform/packages/apps/Dialer/+/699606/

Bug: 109765937
Test: manual
PiperOrigin-RevId: 200096438
Change-Id: Ibecda05fe93edd0a5bbd3bcc29107fc51d3ae2cc