Fix NPE when setting bubble bar bounds

The bubble bar bounds change listener is sometimes invoked before the IBubbles service has been initialized.
This change adds a null check before calling the service.

Flag: ACONFIG com.android.wm.shell.enable_bubble_bar DEVELOPMENT
Fixes: 332423960
Test: manual
Change-Id: Iaa048c62a60c90c5e416da05ba53467a5034d86c
1 file changed