Set language level 1.8 correctly also to Kotlin compiler,

removed unused variables.

core/library/build.gradle

@@ -6,6 +6,13 @@ android {
         baselineFile file("lint-baseline.xml")
         abortOnError true
     }
+    compileOptions {
+        sourceCompatibility JavaVersion.VERSION_1_8
+        targetCompatibility JavaVersion.VERSION_1_8
+    }
+    kotlinOptions {
+        jvmTarget = '1.8'
+    }
 }
 
 dependencies {

core/subsonic-api-image-loader/build.gradle

@@ -4,6 +4,13 @@ android {
     buildFeatures {
         buildConfig = true
     }
+    compileOptions {
+        sourceCompatibility JavaVersion.VERSION_1_8
+        targetCompatibility JavaVersion.VERSION_1_8
+    }
+    kotlinOptions {
+        jvmTarget = '1.8'
+    }
 }
 
 dependencies {

gradle_scripts/android-module-bootstrap.gradle

@@ -17,7 +17,12 @@ android {
         targetSdkVersion versions.targetSdk
     }
 
+    kotlinOptions {
+        jvmTarget = "1.8"
+    }
+
     compileOptions {
+        // Sets Java compatibility to Java 8
         sourceCompatibility JavaVersion.VERSION_1_8
         targetCompatibility JavaVersion.VERSION_1_8
     }
@@ -44,8 +49,6 @@ android {
     }
 }
 
-sourceCompatibility = JavaVersion.VERSION_1_8
-targetCompatibility = JavaVersion.VERSION_1_8
 
 tasks.withType(Test) {
     useJUnitPlatform()

gradle_scripts/kotlin-module-bootstrap.gradle

@@ -3,9 +3,6 @@ apply plugin: 'kotlin'
 apply plugin: 'jacoco'
 apply from: "${project.rootDir}/gradle_scripts/code_quality.gradle"
 
-sourceCompatibility = JavaVersion.VERSION_1_8
-targetCompatibility = JavaVersion.VERSION_1_8
-
 sourceSets {
     main.java.srcDirs += "${projectDir}/src/main/kotlin"
     test.java.srcDirs += "${projectDir}/src/test/kotlin"

ultrasonic/build.gradle

@@ -54,6 +54,10 @@ android {
     kotlinOptions {
         jvmTarget = "1.8"
     }
+    compileOptions {
+        sourceCompatibility JavaVersion.VERSION_1_8
+        targetCompatibility JavaVersion.VERSION_1_8
+    }
 }
 
 tasks.withType(Test) {