[Taskbar] Fix leaked thread "TaskbarUiThread" that is not destroyed upon taskbar recreation.

We use to create a "TaskbarUiThread" for each new taskbar context but forget to destroy the thread.

We fix it by creating a static thread named as "TASKBAR_UI_THREAD"

Fix: TBD
Flag: com.android.launcher3.enable_taskbar_ui_thread
Test: manual
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:66e79baa21573579802ceb7e283c5bb5954a4365)
Merged-In: Idd54c76c39b32a2765c64da1e519c9faa79dac75
Change-Id: Idd54c76c39b32a2765c64da1e519c9faa79dac75
1 file changed