Fix crash on single-core devices and ProGuard rules

app/build.gradle

@@ -67,7 +67,6 @@ android {
 
 dependencies {
     implementation project(':cert4android')
-    implementation ':dav4jvm'
     implementation project(':ical4android')
     implementation project(':vcard4android')
 
@@ -79,6 +78,9 @@ dependencies {
     implementation 'androidx.preference:preference:1.0.0'
     implementation 'com.google.android.material:material:1.0.0'
 
+    implementation(':dav4jvm') {
+        exclude group: 'org.ogce', module: 'xpp3'	// Android comes with its own XmlPullParser
+    }
     implementation 'com.jaredrummler:colorpicker:1.0.5'
     implementation 'com.mikepenz:aboutlibraries:6.2.0'
 

app/src/main/java/at/bitfire/davdroid/syncadapter/SyncManager.kt

@@ -67,8 +67,10 @@ abstract class SyncManager<ResourceType: LocalResource<*>, out CollectionType: L
 
     companion object {
 
-        val MAX_PROCESSING_THREADS = Math.min(Runtime.getRuntime().availableProcessors()/2, 1)
-        val MAX_DOWNLOAD_THREADS = Math.max(Runtime.getRuntime().availableProcessors(), 2)
+        val MAX_PROCESSING_THREADS =    // nCPU/2 (rounded up for case of 1 CPU), but max. 4
+                Math.min((Runtime.getRuntime().availableProcessors()+1)/2, 4)
+        val MAX_DOWNLOAD_THREADS =      // one (if one CPU), 2 otherwise
+                Math.min(Runtime.getRuntime().availableProcessors(), 2)
         const val MAX_MULTIGET_RESOURCES = 10
 
         fun cancelNotifications(manager: NotificationManagerCompat, authority: String, account: Account) =
@@ -79,6 +81,10 @@ abstract class SyncManager<ResourceType: LocalResource<*>, out CollectionType: L
 
     }
 
+    init {
+        Logger.log.info("SyncManager: using up to $MAX_PROCESSING_THREADS processing threads and $MAX_DOWNLOAD_THREADS download threads")
+    }
+
     private val mainAccount = if (localCollection is LocalAddressBook)
         localCollection.mainAccount
     else