Remove monet test folder from SystemUI

The tests are in google3/javatests/com/google/ux/material/libmonet.
There should be no more monet source code in in frameworks/base.

Bug: 290464773
Test: treehugger
Test: atest SystemUITest
Change-Id: Ia687812bd2c48f2494dd1e4bcdbdddbdd9227b84