Delay import of win32clipboard

Under python 2.7.8 on Windows, the default test loader for 'tests' tries
to instantiate a module for each file called '*_tests.py', which fails
for the module 'ui_tests' when pywin32 is not installed.  This causes a
backtrace in setup.py before testing commences.  ui_tests is not part of
the normal test suite; instead of testing, it prints a message saying it
must be run outside of the test suite.

pywin32 does not appear easy to set as a test dependency, as setup fails
to find a suitable egg to build, and pywin32 doesnt provide a pypi
package, wheel binary packages, etc.

Moving the import to the code which uses win32clipboard allows the module
to be instantiated, so it can report a test skip message during testing.

This change allows the tests to be run on MS Windows without
pywin32 or pywinauto installed.

Bug: 68847

Change-Id: I507acbde9f682b9a92a902e9ce22a8d6a64ca583
Merge "Delay import of win32clipboard"