[Partial Screen Sharing] Migrate task switcher flag to aconfig

In order to launch the feature in V, we will have to use the
new flagging system (aconfig).

Flag: ACONFIG com.android.systemui.pss_task_switcher DISABLED
Test: MediaProjectionTaskSwitcherCoreStartableTest.kt
Test: Manual
Bug: 317208379
Fixes: 288868056
Change-Id: I8860987c5188d34178dc27b476907bd6bbf8a6d4