[Android] Fix text selection menu being auto-dismissed for cordova apps

In cordova webview, it will load url like "javascript:js code" for
BridgeMode.
In previous xwalk implementation, the load operation will trigger
a pair of clear/request focus operation. Which makes the text selection
context menu hide immediately after it shows.

The fix is to only request focus when loading url.

BUG=https://github.com/otcshare/cordova-xwalk-android/issues/45