[Tizen] Fix Glib assertion due to appcore_context.

This patch get rid of following log:
[ERROR:browser_main_loop.cc(228)] GLib: g_source_destroy: assertion `source != NULL' failed

AppcoreContext::Initialize() fails when we launch xwalk in the shell. e.g. > xwalk
It is because appcore_init() can success only if registered application (e.g.
web applications) is launched.

This patch handles the exception that AppcoreContext::Initialize() fails.
[Android] Fix #705 to support multiple frames for extension mechanism

When there are multiple frames in one page, extension instance will
be created for each frame. But as internal WebFrame is not exposed
in Java side, there is no way for Java side to identify which
instance to send message. This patch exposed instanceID up to Java
side when it receives message which can be used to identify the
target instance when post message to JavaScript side. What's more
a new interface "broadcastMessage" was added to meet the requirement
that Java side may need to broadcast message to all exiting instances.

Changed interfaces for Java extension:
1. postMessage(String msg) -> postMessage(int instanceID, String msg)
2. onMessage(String msg)   -> onMessage(int instanceID, String msg)
3. onSyncMessage(String msg) -> onSyncMessage(int instanceID, String msg)
4. new added: broadcastMessage(String msg)

BUG=#705
[Android] Fix text selection menu being auto-dismissed in some cases

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 sometimes.

The fix is to only request focus when loading url.
Merge pull request #950 from wang16/cordova_txt_sel

[Android] Fix text selection menu being auto-dismissed in some cases
Merge pull request #935 from ds-hwang/appcorebug

[Tizen] Fix Glib assertion due to appcore_context.
Merge pull request #907 from DonnaWuDongxia/multi-frame-support-in-one

[Android] Fix #705 to support multiple frames for extension mechanism
[Android] Define the class XWalkRuntimeViewProvider as an interface.

It's an abstract class and some shared implementations are included
and will be used by inherited classes. However, xwalk core library needs
an interface which will be implemented by the container.
To meet this requirement, split this class into two: one interface containing
all methods and the base class to contain some shared implementations like
managing extensions.