Use IconProvider to get application icon.

Changes logic to get app icon to use IconProvider. Also places it such
that icon and name are fetched on WindowDecor creation rather than every
time handle menu is built.

Bug: 268363572
Test: Manual, ensure correct icon/name are still fetched.
Change-Id: I546fbead9b4d86a3fae61e1a21814df57f8363a2