Unmark flakiness for PipKeyboardTestShellTransit

This doesn't appear to be flaky anymore (see dashboard), so
unmarking flaky.

Unfortunately, it looks like non-shell-transit is still marked
flaky, so we can't "unify" the test suite yet.

Bug: 227313015
Test: atest WMShellFlickerTests:PipKeyboardTestShellTransit
Change-Id: I501fab0aa9316a56b944452b66708e4e8c831700