feat(Android): Support Android file upload (#60)

Fixes #33 

I could really use some help from an Android developer on this one, because I just "made it work", don't know how to "make it work good".

Some things that should be reviewed:

- [ ] validate Android 5.0 devices (my emulator work, but outputs some weird sounds; a Galaxy 4 I tested on crashes)
- [ ] validate Android 5.1 devices (emulator works, couldn't find a real device)
- [ ] how to handle File Extensions? (https://www.w3schools.com/tags/att_input_accept.asp)

I'm sure that there's more refactoring to be done, so any help and advice would be appreciated.

android/src/main/java/com/reactnativecommunity/webview/RNCWebViewFileProvider.java

@@ -0,0 +1,14 @@
+package com.reactnativecommunity.webview;
+
+import android.support.v4.content.FileProvider;
+
+/**
+ * Providing a custom {@code FileProvider} prevents manifest {@code <provider>} name collisions.
+ *
+ * See https://developer.android.com/guide/topics/manifest/provider-element.html for details.
+ */
+public class RNCWebViewFileProvider extends FileProvider {
+
+  // This class intentionally left blank.
+
+}