Ensure task description icon resources are loaded from the right package

- We previously assumed that icons set in the task description would be
  loaded from the same package as the root activity (which is incorrect),
  instead, they should be fetched from the activity which set them.

  We also deprecate getIcon() because:
  - apps could only get the icons they set previously (not very useful)
  - it didn't return resource icons, only in memory or loaded from disk
  - the name doesn't indicate that it could result in a disk load
  Instead, callers (namely SysUI) should now call loadIcon() to get the
  actual icon directly (or loaded if necessary).

Bug: 143363444
Test: atest TaskDescriptionTest
Test: atest ActivityManagerTest
Change-Id: I48aaba17de2edf0b3a61fa049cf897020d4c2ef1