Add isInDebug variable to simulate debug/release configs for tests (#2566)

* `isInDebug` as `ThreadLocal<Boolean>`

* Use a simple var for `isInDebug` and add a helper method to test release mode, when running the debug test.

* Add some more docs

---------

Co-authored-by: Benoit Marty <benoit@matrix.org>

libraries/androidutils/build.gradle.kts

@@ -20,6 +20,10 @@ plugins {
 
 android {
     namespace = "io.element.android.libraries.androidutils"
+
+    buildFeatures {
+        buildConfig = true
+    }
 }
 
 anvil {