android: Change how apps access their APK's "assets" directory.

Now they can explicitly access it with "assets://" filenames, and
SDL_GetBasePath() returns "assets://"

Fixes #15347.
Fixes #5044.

src/filesystem/android/SDL_sysfilesystem.c

@@ -31,7 +31,7 @@
 
 char *SDL_SYS_GetBasePath(void)
 {
-    return SDL_strdup("./");
+    return SDL_strdup("assets://");
 }
 
 char *SDL_SYS_GetPrefPath(const char *org, const char *app)