[Bubbles] Fix focus issue by removing bubble after new bubble is added

Before this change, when switching between floating bubbles, we
remove the previous bubble and then add the new bubble. This caused
an intermediate state where there is no focusable bubble, which
caused the non-bubbled activity behind to gain focus and might cause
IME to show. This change ensures that the previous bubble is removed
after the new bubble is added.

Bug: 416347012
Test: atest BubbleStackViewTest
Flag: com.android.wm.shell.fix_bubbles_ime_focus_flicker
Change-Id: Iab6deda1efcbb41d627d75aeac126d12de20212e