Update gradle build script to allow installing debug builds alongside store version

This will allow developers, testers, etc to work on Tusky will not having to worry
about overwriting, uninstalling, fiddling with a preinstalled application which would
mean having to login again every time the development cycle starts/finishes and
manually reinstalling the app.

app/build.gradle

@@ -30,7 +30,9 @@ android {
             shrinkResources true
             proguardFiles 'proguard-rules.pro'
         }
-        debug { }
+        debug {
+            applicationIdSuffix ".debug"
+        }
     }
 
     flavorDimensions "color"