New MP4 muxer + Queue changes + Storage fixes

Main changes:
* correctly check the available space (CircularFile.java)
* misc cleanup (CircularFile.java)
* use the "Error Reporter" for non-http errors
* rewrite network state checking and add better support for API 21 (Lollipop) or higher
* implement "metered networks"
* add buttons in "Downloads" activity to start/pause all pending downloads, ignoring the queue flag or if the network is "metered"
* add workaround for VPN connections and/or network switching. Example: switching WiFi to 3G
* rewrite DataReader ¡Webm muxer is now 57% more faster!
* rewrite CircularFile, use file buffers instead of memory buffers. Less troubles in low-end devices
* fix missing offset for KaxCluster (WebMWriter.java), manifested as no thumbnails on file explorers

Download queue:
* remember queue status, unless the user pause the download (un-queue)
* semi-automatic downloads, between networks. Effective if the user create a new download or the downloads activity is starts
* allow enqueue failed downloads
* new option, queue limit, enabled by default. Used to allow one or multiple downloads at same time

Miscellaneous:
* fix crash while selecting details/error menu (mistake on MissionFragment.java)
* misc serialize changes (DownloadMission.java)
* minor UI tweaks
* allow overwrite paused downloads
* fix wrong icons for grid/list button in downloads
* add share option
* implement #2006
* correct misspelled word in strings.xml (es) (cmn)
* fix MissionAdapter crash during device shutdown

New Mp4Muxer + required changes:
* new mp4 muxer (from dash only) with this, muxing on Android 7 is possible now!!!
* re-work in SharpStream
* drop mp4 dash muxer
* misc changes: add warning in SecondaryStreamHelper.java,
* strip m4a DASH files to normal m4a format (youtube only)

Fix storage issues:
* warn to the user if is choosing a "read only" download directory (for external SD Cards), useless is rooted :)
* "write proof" allow post-processing resuming only if the device ran out of space
* implement "insufficient storage" error for downloads

app/src/main/java/us/shandian/giga/postprocessing/Mp4FromDashMuxer.java

@@ -0,0 +1,29 @@
+package us.shandian.giga.postprocessing;
+
+import org.schabi.newpipe.streams.Mp4FromDashWriter;
+import org.schabi.newpipe.streams.io.SharpStream;
+
+import java.io.IOException;
+
+import us.shandian.giga.get.DownloadMission;
+
+/**
+ * @author kapodamy
+ */
+class Mp4FromDashMuxer extends Postprocessing {
+
+    Mp4FromDashMuxer(DownloadMission mission) {
+        super(mission, 2 * 1024 * 1024/* 2 MiB */, true);
+    }
+
+    @Override
+    int process(SharpStream out, SharpStream... sources) throws IOException {
+        Mp4FromDashWriter muxer = new Mp4FromDashWriter(sources);
+        muxer.parseSources();
+        muxer.selectTracks(0, 0);
+        muxer.build(out);
+
+        return OK_RESULT;
+    }
+
+}