Change how the toolbar spinner total progress is calculated

Before it used a byte count, but byte counts aren't available for queued
items. So just use an average of the percents instead.

Bug: 400352300
Test: atest -c 'DocumentsUIGoogleTests:com.android.documentsui.JobPanelControllerTest'
Flag: com.android.documentsui.flags.visual_signals_ro
Change-Id: I0625e04ad17eef7167a33388318b92336a2e3f87