Use streams to encode and decode the JSON backups

This significantly reduces memory usage.

(We buffer the OutputStream because kotlinx.serialization flushes its
internal buffers excessively[1]. We also buffer the InputStream for
consistency, even though kotlinx.serialization uses adequate
buffering; there is no performance impact because BufferedInputStream
cascades harmlessly[2].)

The functions encodeToStream() and decodeFromStream() are marked as
experimental, but this is a pretty fundamental use case so surely it
will continue to be supported in the future (maybe with minor
changes).

Fixes #6.

[1] https://github.com/Kotlin/kotlinx.serialization/blob/v1.6.3/formats/json/jvmMain/src/kotlinx/serialization/json/internal/JvmJsonStreams.kt#L46
[2] https://github.com/openjdk/jdk/blob/jdk-23%2B14/src/java.base/share/classes/java/io/BufferedInputStream.java#L339

app/src/main/kotlin/org/fossify/messages/activities/SettingsActivity.kt

@@ -7,8 +7,8 @@ import android.os.Build
 import android.os.Bundle
 import android.provider.DocumentsContract
 import androidx.activity.result.contract.ActivityResultContracts
-import kotlinx.serialization.encodeToString
 import kotlinx.serialization.json.Json
+import kotlinx.serialization.json.encodeToStream
 import org.fossify.commons.activities.ManageBlockedNumbersActivity
 import org.fossify.commons.dialogs.*
 import org.fossify.commons.extensions.*
@@ -118,6 +118,7 @@ class SettingsActivity : SimpleActivity() {
         }
     }
 
+    @OptIn(kotlinx.serialization.ExperimentalSerializationApi::class)
     private fun exportMessages(uri: Uri) {
         ensureBackgroundThread {
             var success = false
@@ -128,11 +129,8 @@ class SettingsActivity : SimpleActivity() {
                         return@getMessagesToExport
                     }
                     val json = Json { encodeDefaults = true }
-                    val jsonString = json.encodeToString(messagesToExport)
+                    contentResolver.openOutputStream(uri)!!.buffered().use { outputStream ->
-                    val outputStream = contentResolver.openOutputStream(uri)!!
+                        json.encodeToStream(messagesToExport, outputStream)
-
-                    outputStream.use {
-                        it.write(jsonString.toByteArray())
                     }
                     success = true
                     toast(org.fossify.commons.R.string.exporting_successful)

app/src/main/kotlin/org/fossify/messages/helpers/MessagesImporter.kt

@@ -4,6 +4,7 @@ import android.net.Uri
 import android.util.Xml
 import kotlinx.serialization.SerializationException
 import kotlinx.serialization.json.Json
+import kotlinx.serialization.json.decodeFromStream
 import org.fossify.commons.extensions.showErrorToast
 import org.fossify.commons.extensions.toast
 import org.fossify.commons.helpers.ensureBackgroundThread
@@ -37,13 +38,12 @@ class MessagesImporter(private val activity: SimpleActivity) {
         }
     }
 
+    @OptIn(kotlinx.serialization.ExperimentalSerializationApi::class)
     private fun importJson(uri: Uri) {
         try {
-            val jsonString = activity.contentResolver.openInputStream(uri)!!.use { inputStream ->
+            val deserializedList = activity.contentResolver.openInputStream(uri)!!.buffered().use { inputStream ->
-                inputStream.bufferedReader().readText()
+                Json.decodeFromStream<List<MessagesBackup>>(inputStream)
             }
-
-            val deserializedList = Json.decodeFromString<List<MessagesBackup>>(jsonString)
             if (deserializedList.isEmpty()) {
                 activity.toast(org.fossify.commons.R.string.no_entries_for_importing)
                 return