Disable failing pip assertion

Test is very flaky in presubmit. Disable it until we can identify what is causing the failure. Currently sponge cannot fetch tests results and, thus, cannot be debugged.

Bug: 193436187

Test: atest WMShellFlickerTests
Change-Id: Ieb7a80d0675aefe1a80d10a0f8b512506d063bba