Restricts callers to fetch activity icons for their own apps

- With the exception of SysUI, callers should only be able
  to fetch icons set on their own task descriptions
- Removing unused code to fetch task description for a given task

Bug: 318087006
Test: atest ActivityTaskManagerServiceTests
Test: Verify with POC apks in the bug
Change-Id: Ie7822e5701064fc3bc2e3cfb94c541ea21d02228