simpleperf: fix importing tkinter in report.py.

On darwin/windows, python2.7 doesn't have tkinter.
So we need to try both tkinter and Tkinter.

Also make the font size larger. Because the old size
is too small on darwin.

Bug: http://b/32834638
Test: run report.py on linux/darwin/windows.
Change-Id: I8bb0b51fcdd99f70b01d4ca2ee2e9e9a6447870a
1 file changed