Added support for asset loading on Android.

Requires passing the activity's AssetManager object from Java to GNUstep by calling +[NSBundle setJavaAssetManager:withJNIEnv:], which then enables the following features:

- NSBundle main bundle resource paths support for Android assets, e.g. for pathForResource:ofType:, URLForResource:ofType: and related methods.
- NSBundle main bundle info dictionary support if Info.plist exists in Android assets.
- -initWithContentsOfFile: and related methods support for reading Android assets from main bundle in various classes (e.g. NSData, NSDictionary, NSArray, etc.).
- NSFileManager fileExistsAtPath:(isDirectory:) and isReadableFileAtPath: return YES for main bundle asset / asset directory paths.
- NSFileHandle support for reading Android assets from main bundle.
- NSDirectoryEnumerator support for enumerating Android assets from main bundle. Note that recursion into subdirectories is currently not supported by the native Android asset manager API (see https://issuetracker.google.com/issues/37002833).

Also adds support for automatic NSProcessInfo initialization on Android with a fake executable path "/data/data/<app identifier>/exe" (as Android apps don't have a real executable path), and tweaks main bundle initialization to allow that path. Main bundle resource paths are prefixed by "/data/data/<app identifier>/Resources".

Source/NSData.m

@@ -253,6 +253,35 @@ readContentsOfFile(NSString *path, void **buf, off_t *len, NSZone *zone)
       NSWarnFLog(@"Open (%@) attempt failed - bad path", path);
       return NO;
     }
+  
+#ifdef __ANDROID__
+  // Android: try using asset manager if path is in main bundle resources
+  AAsset *asset = [NSBundle assetForPath:path];
+  if (asset) {
+    fileLength = AAsset_getLength(asset);
+
+    tmp = NSZoneMalloc(zone, fileLength);
+    if (tmp == 0) {
+      NSLog(@"Malloc failed for file (%@) of length %jd - %@", path,
+        (intmax_t)fileLength, [NSError _last]);
+      AAsset_close(asset);
+      goto failure;
+    }
+
+    int result = AAsset_read(asset, tmp, fileLength);
+    AAsset_close(asset);
+    
+    if (result < 0) {
+      NSWarnFLog(@"read of file (%@) contents failed - %@", path,
+        [NSError errorWithDomain:NSPOSIXErrorDomain code:result userInfo:nil]);
+      goto failure;
+    }
+    
+    *buf = tmp;
+    *len = fileLength;
+    return YES;
+  }
+#endif /* __ANDROID__ */
 
   att = [mgr fileAttributesAtPath: path traverseLink: YES];
   if (nil == att)