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/model/ObservableTunnel.kt

@@ -11,9 +11,9 @@ import com.wireguard.android.BR
 import com.wireguard.android.backend.Statistics
 import com.wireguard.android.backend.Tunnel
 import com.wireguard.android.databinding.Keyed
+import com.wireguard.android.util.applicationScope
 import com.wireguard.config.Config
 import kotlinx.coroutines.Dispatchers
-import kotlinx.coroutines.GlobalScope
 import kotlinx.coroutines.launch
 import kotlinx.coroutines.withContext
 
@@ -74,7 +74,7 @@ class ObservableTunnel internal constructor(
         get() {
             if (field == null)
             // Opportunistically fetch this if we don't have a cached one, and rely on data bindings to update it eventually
-                GlobalScope.launch(Dispatchers.Main.immediate) {
+                applicationScope.launch {
                     try {
                         manager.getTunnelConfig(this@ObservableTunnel)
                     } catch (e: Throwable) {
@@ -110,7 +110,7 @@ class ObservableTunnel internal constructor(
         get() {
             if (field == null || field?.isStale != false)
             // Opportunistically fetch this if we don't have a cached one, and rely on data bindings to update it eventually
-                GlobalScope.launch(Dispatchers.Main.immediate) {
+                applicationScope.launch {
                     try {
                         manager.getTunnelStatistics(this@ObservableTunnel)
                     } catch (e: Throwable) {