[Android] Fix potential memory leak in XWalkViewInternal

Use WeakReference to register ActivityStateListener.
Otherwise, the ActivityStateListener will hold
a reference of XWalkViewInternal as in static observer
list.
Bump version to 8.36.173.0
Merge pull request #2198 from wang16/fix_activity_listener_leak

[Android] Fix potential memory leak in XWalkViewInternal
[Android]

"version" field in manifest should not be mandatory and should be renamed to "xwalk_version"

According W3C manifest spec, 'version' field of manifest should be:
1) renamed to "xwalk_version"
2) optional
3) the "version" field should be handled if present, but issue a deprecation warning

BUG=https://crosswalk-project.org/jira/browse/XWALK-2053