Change logic to check recents bookend transition flag

- We are separating this from the top task tracking since they can land
  independently

Test: atest WMShellUnitTests
Flag: com.android.wm.shell.enable_recents_bookend_transition
Bug: 346588978

Change-Id: I34231086f89fe86cad0121bdde0d7cd73f0e9542
Signed-off-by: Winson Chung <winsonc@google.com>