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".

Headers/Foundation/NSBundle.h

@@ -36,6 +36,10 @@ extern "C" {
 #import	<Foundation/NSObject.h>
 #import	<Foundation/NSString.h>
 
+#ifdef __ANDROID__
+#include <android/asset_manager_jni.h>
+#endif
+
 @class NSString;
 @class NSArray;
 @class NSDictionary;
@@ -540,6 +544,38 @@ GS_EXPORT NSString* const NSLoadedClasses;
 			      ofType: (NSString*)extension
 			 inDirectory: (NSString*)bundlePath;
 
+/** Cleans up the path cache for the bundle. */
+- (void) cleanPathCache;
+
+#ifdef __ANDROID__
+
+/**
+ * Sets the Java Android asset manager.
+ * The developer can call this method to enable asset loading via NSBundle.
+ */
++ (void) setJavaAssetManager:(jobject)jassetManager withJNIEnv:(JNIEnv *)env;
+
+/**
+ * Returns the native Android asset manager.
+ */
++ (AAssetManager *) assetManager;
+
+/**
+ * Returns the Android asset for the given path if path is in main bundle
+ * resources and asset exists.
+ * The returned object must be released using AAsset_close().
+ */
++ (AAsset *)assetForPath:(NSString *)path;
+
+/**
+ * Returns the Android asset dir for the given path if path is in main bundle
+ * resources and the asset directory exists.
+ * The returned object must be released using AAssetDir_close().
+ */
++ (AAssetDir *)assetDirForPath:(NSString *)path;
+
+#endif /* __ANDROID__ */
+
 @end
 
 #endif /* GNUSTEP */