Implement Storage Access Framework

* re-work finished mission database
* re-work DownloadMission and bump it Serializable version
* keep the classic Java IO API
* SAF Tree API support on Android Lollipop or higher
* add wrapper for SAF stream opening
* implement Closeable in SharpStream to replace the dispose() method

* do required changes for this API:
** remove any file creation logic from DownloadInitializer
** make PostProcessing Serializable and reduce the number of iterations
** update all strings.xml files
** storage helpers: StoredDirectoryHelper & StoredFileHelper
** best effort to handle any kind of SAF errors/exceptions

app/src/main/java/us/shandian/giga/io/SharpInputStream.java

@@ -0,0 +1,61 @@
+/*
+ * To change this license header, choose License Headers in Project Properties.
+ * To change this template file, choose Tools | Templates
+ * and open the template in the editor.
+ */
+package us.shandian.giga.io;
+
+import android.support.annotation.NonNull;
+
+import org.schabi.newpipe.streams.io.SharpStream;
+
+import java.io.IOException;
+import java.io.InputStream;
+
+/**
+ * Wrapper for the classic {@link java.io.InputStream}
+ *
+ * @author kapodamy
+ */
+public class SharpInputStream extends InputStream {
+
+    private final SharpStream base;
+
+    public SharpInputStream(SharpStream base) throws IOException {
+        if (!base.canRead()) {
+            throw new IOException("The provided stream is not readable");
+        }
+        this.base = base;
+    }
+
+    @Override
+    public int read() throws IOException {
+        return base.read();
+    }
+
+    @Override
+    public int read(@NonNull byte[] bytes) throws IOException {
+        return base.read(bytes);
+    }
+
+    @Override
+    public int read(@NonNull byte[] bytes, int i, int i1) throws IOException {
+        return base.read(bytes, i, i1);
+    }
+
+    @Override
+    public long skip(long l) throws IOException {
+        return base.skip(l);
+    }
+
+    @Override
+    public int available() {
+        long res = base.available();
+        return res > Integer.MAX_VALUE ? Integer.MAX_VALUE : (int) res;
+    }
+
+    @Override
+    public void close() {
+        base.close();
+    }
+}