Refactor: Extract all cross-platform code from CustomSaveLocationHelpers into core module (#6962)

* Also fixes the GameInfo.customSaveLocation to work for Android

android/src/com/unciv/app/CustomFileLocationHelperAndroid.kt

@@ -0,0 +1,98 @@
+package com.unciv.app
+
+import android.app.Activity
+import android.content.Intent
+import android.net.Uri
+import android.provider.DocumentsContract
+import android.provider.OpenableColumns
+import androidx.annotation.GuardedBy
+import com.unciv.logic.CustomFileLocationHelper
+import com.unciv.ui.crashhandling.postCrashHandlingRunnable
+import java.io.InputStream
+import java.io.OutputStream
+
+class CustomFileLocationHelperAndroid(private val activity: Activity) : CustomFileLocationHelper() {
+
+    @GuardedBy("this")
+    private val callbacks = mutableListOf<ActivityCallback>()
+    @GuardedBy("this")
+    private var curActivityRequestCode = 100
+
+    override fun createOutputStream(suggestedLocation: String, callback: (String?, OutputStream?, Exception?) -> Unit) {
+        val requestCode = createActivityCallback(callback) { activity.contentResolver.openOutputStream(it, "rwt") }
+
+        // When we loaded, we returned a "content://" URI as file location.
+        val uri = Uri.parse(suggestedLocation)
+        val fileName = if (uri.scheme == "content") {
+            val cursor = activity.contentResolver.query(uri, null, null, null, null)
+            cursor.use {
+                // we should have a direct URI to a file, so first is enough
+                if (it?.moveToFirst() == true) {
+                    it.getString(it.getColumnIndex(OpenableColumns.DISPLAY_NAME))
+                } else {
+                    ""
+                }
+            }
+        } else {
+            // if we didn't load, this is some file name entered by the user
+            suggestedLocation
+        }
+
+        Intent(Intent.ACTION_CREATE_DOCUMENT).apply {
+            type = "application/json"
+            putExtra(Intent.EXTRA_TITLE, fileName)
+            if (uri.scheme == "content") {
+                putExtra(DocumentsContract.EXTRA_INITIAL_URI, uri)
+            }
+            activity.startActivityForResult(this, requestCode)
+        }
+    }
+
+    override fun createInputStream(callback: (String?, InputStream?, Exception?) -> Unit) {
+        val callbackIndex = createActivityCallback(callback, activity.contentResolver::openInputStream)
+
+        Intent(Intent.ACTION_OPEN_DOCUMENT).apply {
+            type = "*/*"
+            // It is theoretically possible to use an initial URI here, however, the only Android URIs we have are obtained from here, so, no dice
+            activity.startActivityForResult(this, callbackIndex)
+        }
+    }
+
+    private fun <T> createActivityCallback(callback: (String?, T?, Exception?) -> Unit,
+                                           createValue: (Uri) -> T): Int {
+        synchronized(this) {
+            val requestCode = curActivityRequestCode++
+            val activityCallback = ActivityCallback(requestCode) { uri ->
+                if (uri == null) {
+                    callback(null, null, null)
+                    return@ActivityCallback
+                }
+
+                try {
+                    val outputStream = createValue(uri)
+                    callback(uri.toString(), outputStream, null)
+                } catch (ex: Exception) {
+                    callback(null, null, ex)
+                }
+            }
+            callbacks.add(activityCallback)
+            return requestCode
+        }
+    }
+
+    fun onActivityResult(requestCode: Int, data: Intent?) {
+        val callback = synchronized(this) {
+            val index = callbacks.indexOfFirst { it.requestCode == requestCode }
+            if (index == -1) return
+            callbacks.removeAt(index)
+        }
+        postCrashHandlingRunnable {
+            callback.callback(data?.data)
+        }
+    }
+}
+
+private class ActivityCallback(
+    val requestCode: Int,
+    val callback: (Uri?) -> Unit
+)