[Android] Fix that dict.iteritems() is no longer supported in python 3.0+

'dict' object has no attribute 'iteritems' in python 3.0 and later,
use dict.items() instead.

BUG=https://crosswalk-project.org/jira/browse/XWALK-2129
Merge pull request #2213 from jinbang/fix_dict_iteritems

[Android] Fix that dict.iteritems() is no longer supported in python 3.0...
[Android] make_apk: Fail early if --icon does not exist.

Fail as early as possible if the icon passed via the --icon option does
not exist, not when it needs to be copied elsewhere.
[Android] make_apk: Stop passing icon_dict everywhere.

Add a small amount of sanity to make_apk's code and consolidate the
value of the "icon" option/property in a single place, AppInfo.

Instead of keeping two separate variables, one in AppInfo with the value
passed to --icon and a separate one with the value from the manifest,
just set AppInfo's |icon| member with --icon if it is set, and "icon"
from the manifest otherwise (as the current logic does).
[Android] make_apk: Make CustomizeIconByDict() more readable.

Get rid of the convoluted code that did lots of needless operations on
the |drawable_dict| array and just do checks on ranges instead to get
the size name corresponding to a given size in the manifest.