manually use aapt singleCrunch for reproducible builds

PNG crunching is not a deterministic process, especially the way aapt does
it.  This makes the F-Droid builds not reproducible.  The easy solution to
this is to pre-crunch the PNGs and commit them to git.  It also makes the
final APK a tiny amount smaller, for whatever reason.

https://medium.com/@duhroach/smaller-pngs-and-android-s-aapt-tool-4ce38a24019d

app/build.gradle

@@ -207,6 +207,10 @@ android {
         targetCompatibility JavaVersion.VERSION_1_7
     }
 
+    aaptOptions {
+        cruncherEnabled = false
+    }
+
     defaultConfig {
         versionCode 1000001
         versionName getVersionName()