ui: replace GlobalScope with a hand-rolled CoroutineScope

GlobalScope has numerous problems[1] that make it unfit for
use in most applications and making it behave correctly requires
an excessive amount of verbosity that's alleviated simply by using
any other scope. Since we run multiple operations in the context
of the application's lifecycle, introduce a new scope that is created
when our application is, and cancelled upon its termination.

While at it, make the scope default to Dispatchers.IO to reduce pressure
on the UI event loop. Tasks requiring access to the UI thread appropriately
switch context making the change completely safe.

1: https://medium.com/@elizarov/the-reason-to-avoid-globalscope-835337445abc

Signed-off-by: Harsh Shandilya <me@msfjarvis.dev>

ui/src/main/java/com/wireguard/android/util/Extensions.kt

@@ -11,6 +11,7 @@ import androidx.annotation.AttrRes
 import androidx.fragment.app.Fragment
 import androidx.lifecycle.lifecycleScope
 import androidx.preference.Preference
+import com.wireguard.android.Application
 import com.wireguard.android.activity.SettingsActivity
 import kotlinx.coroutines.CoroutineScope
 
@@ -24,6 +25,9 @@ fun Fragment.requireTargetFragment(): Fragment {
     return requireNotNull(targetFragment) { "A target fragment should always be set for $this" }
 }
 
+val Any.applicationScope: CoroutineScope
+    get() = Application.getCoroutineScope()
+
 val Preference.activity: SettingsActivity
     get() = context as? SettingsActivity ?: throw IllegalStateException("Failed to resolve SettingsActivity")