Use File extension functions to make code more concise. (#1042)

Co-authored-by: Jakob Möller <jakob.moeller@sap.com>

Corona-Warn-App/src/main/java/de/rki/coronawarnapp/util/KeyFileHelper.kt

@@ -12,10 +12,7 @@ import kotlinx.coroutines.async
 import kotlinx.coroutines.awaitAll
 import kotlinx.coroutines.withContext
 import timber.log.Timber
-import java.io.BufferedWriter
 import java.io.File
-import java.io.FileOutputStream
-import java.io.OutputStreamWriter
 import java.nio.charset.Charset
 import java.util.Date
 import java.util.UUID
@@ -126,7 +123,7 @@ object KeyFileHelper {
 
     private fun TEKSignatureList.writeToFile(
         file: File
-    ) = FileOutputStream(file).use { stream ->
+    ) = file.outputStream().use { stream ->
         this.writeTo(stream)
     }
 
@@ -144,9 +141,9 @@ object KeyFileHelper {
 
     private fun getExportBinaryFileName(): String = "key-export-binary-${UUID.randomUUID()}.bin"
 
-    private fun File.appendBinaryHeader() = FileOutputStream(this).use { fos ->
-        OutputStreamWriter(fos, Charset.forName(EXPORT_FILE_HEADER_CHARSET)).use { osw ->
-            BufferedWriter(osw).use { bw ->
+    private fun File.appendBinaryHeader() = outputStream().use { fos ->
+        fos.writer(Charset.forName(EXPORT_FILE_HEADER_CHARSET)).use { osw ->
+            osw.buffered().use { bw ->
                 bw.write(EXPORT_FILE_HEADER)
             }
         }

Corona-Warn-App/src/main/java/de/rki/coronawarnapp/util/ZipHelper.kt

@@ -22,7 +22,6 @@ package de.rki.coronawarnapp.util
 import timber.log.Timber
 import java.io.File
 import java.io.FileInputStream
-import java.io.FileOutputStream
 import java.io.InputStream
 import java.util.zip.ZipEntry
 import java.util.zip.ZipInputStream
@@ -44,7 +43,7 @@ object ZipHelper {
         }
         outputFile.createNewFile()
 
-        FileOutputStream(outputFile).use { fileOutputStream ->
+        outputFile.outputStream().use { fileOutputStream ->
             ZipOutputStream(fileOutputStream).use { zipOutputStream ->
                 writeToZip(
                     nameOfOutputFile,