Fix DesktopModeWindowDecor leak on last bubble removal.

This change fixes a performance regression (b/412782113) in the bubble
cleanup process that caused desktop mode window decoration
"leaks" during bubble removal. The issue particularly impacts the
ShowMultipleBubblesAndSwitchMicrobenchmark test, showing regressions
of over 1,700% in missed frames during SPLASHSCREEN_EXIT_ANIM.

The problem occurs during the removal of the last bubble from the stack,
where two window container transitions (WCT) are triggered in sequence:
a TO_BACK transition followed by a CLOSE transition. By the time the
TO_BACK transition occurs, the bubble is already removed from the bubble
data, causing AppHandleAndHeaderVisibilityHelper to incorrectly
allow desktop mode window decorations for the removed bubble task.

This change eliminates the unnecessary TO_BACK transition when removing
the last bubble from an empty stack, allowing the CLOSE transition to
handle the task removal directly. This prevents the accumulation of
desktop decorations and significantly improves performance metrics.

Bug: 416655338
Bug: 408389476
Bug: 388630258
Flag: com.android.wm.shell.enable_create_any_bubble
Flag: com.android.window.flags.exclude_task_from_recents
Test: atest WMShellRobolectricTests:BubbleControllerTest
Test: atest systemui-bubble-1-jank-suite \
   --request-upload-result \
   -- --enable-module-dynamic-download \
   --module-arg systemui-bubble-1-jank-suite:strict-include-metric-filter:'perfetto_ft_systemui-missed_app_frames-mean' \
   --test-arg com.android.tradefed.testtype.AndroidJUnitTest:class:android.platform.test.scenario.sysui.bubble.ShowMultipleBubblesAndSwitchMicrobenchmark
http: //ab/I4940001039473788 (flag enabled, SPLASHSCREEN_EXIT_ANIM-counter_metrics-missed_app_frames-mean:0.04)
Change-Id: Ia37433a8cd6c2cfb7187cb171e791de6e1b77122