Enable desugaring (needed by ical4android).

ical4j and thus ical4android now rely on java.time.ZoneOffset which is only available
in Android starting from Android 8. This will make it work on older Android versions
too.

app/build.gradle

@@ -122,6 +122,9 @@ android {
         buildTypes.release.signingConfig = null
     }
     compileOptions {
+        // enable because ical4android requires desugaring
+        coreLibraryDesugaringEnabled true
+
         sourceCompatibility JavaVersion.VERSION_1_8
         targetCompatibility JavaVersion.VERSION_1_8
     }
@@ -135,6 +138,8 @@ android {
 
 dependencies {
     implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"
+    coreLibraryDesugaring 'com.android.tools:desugar_jdk_libs:1.0.5'
+
     implementation "org.jetbrains.anko:anko-commons:0.10.4"
 
     implementation "com.etesync:journalmanager:1.1.1"

app/proguard-rules.txt

@@ -10,7 +10,6 @@
 -optimizations !code/simplification/arithmetic,!code/simplification/cast,!field/*,!class/merging/*
 -optimizationpasses 5
 -allowaccessmodification
--dontpreverify
 
 # Kotlin
 -dontwarn kotlin.**
@@ -25,14 +24,7 @@
 -keep class ezvcard.property.** { *; }  # keep all vCard properties (created at runtime)
 
 # ical4j: ignore unused dynamic libraries
--dontwarn aQute.**
--dontwarn groovy.**                       # Groovy-based ContentBuilder not used
--dontwarn javax.cache.**                  # no JCache support in Android
--dontwarn net.fortuna.ical4j.model.**
--dontwarn org.codehaus.groovy.**
--dontwarn org.apache.log4j.**             # ignore warnings from log4j dependency
 -keep class net.fortuna.ical4j.** { *; }  # keep all model classes (properties/factories, created at runtime)
--keep class org.threeten.bp.** { *; }     # keep ThreeTen (for time zone processing)
 
 # okhttp
 # JSR 305 annotations are for embedding nullability information.