[Android] Fix app crash that two page load each other

The crash is local reference table overflow (max=512), it's a thread hold the input stream reference by analysing MAT.
Threads attached through JNI must cal DetachCurrentThread before they exit. So call DetachFromVM after AttachCurrentThread which is wrap the DetachCurrentThread function.

BUG=XWALK-3096
why not calling it after `PostTask`?