Embedding api v2

This commit added following APIs as Embedding API:
XWalkResourceClient.shouldOverrideURLLoading
XWalkUIClient.onUnhandledKeyEvent
XWalkUIClient.shouldOverrideKeyEvent
XWalkUIClient.onReceivedTitle
XWalkUIClient.onPageLoadStarted
XWalkUIClient.onPageLoadStopped
XWalkUIClient.onCreateWindowRequested

Some of those APIs were implemented inside xwalk core
internal previously. Now moving them to public Client
classes of XWalkView to expose in Embedding API.

Beside, remove the internal callbacks which are decided
not to expose in Embedding API and currently are dead code
or have empty implementation in core internal, including:
XWalkWebChromeClient.onReceivedTouchIconUrl
XWalkWebChromeClient.getDefaultVideoPoster
XWalkWebChromeClient.getVideoLoadingProgressView

TODO(wang16): Implement XWalkUIClient.onCreateWindowRequested
and LoadStatus for XWalkUIClient.onPageLoadStopped.

Related BUGs: XWALK-2005, XWALK-2031
BUG=XWALK-2003, XWALK-2004