[DocsUI M3] Use font unit for breadcrumb arrow icon size

This is to make sure the arrow icon size in the breadcrumb will
change together with system font/display size change.

Check the attached bug for the screenshot comparison.

Bug: 437292083
Test: m DocumentsUIGoogle && change font size and check arrow icon
Flag: com.android.documentsui.flags.use_material3
Change-Id: Iae2154e0f4bcafe6fb4f09b6c7f5edb1cfa6a756