Apps can opt in to be stoppable in the task manager

Before this change, a system apps having foreground service wasn't
stoppable in the task manager. The rationale for it was that users being
able to stop system apps can be risky. However, that at the same time
meant that users have no way to stop such apps even if the apps can be
stopped without any risk.

This change adds an array stoppable_fgs_system_apps, which device makers
can put the package name of the system apps can be safely stopped.

Bug: 376564917
Test: watch SystemUITests in TH
Test: check if there is a stop button in the UI with flag and config
Flag: com.android.systemui.stoppable_fgs_system_app
Change-Id: I09a946fdcba695f2ae0e0da333cce1e43c7c7421