Cancel pending tasks by callling underTest.onPause to avoid mockito-npe

KeyguardPinViewController extends KeyguardAbsKeyInputViewController
which can have mPendingLockCheck. Call #onPause to cancel the pending
lock check so we don't have it running after we finish running
the test and the mocks are null (causing npe).

Fixes: 424667843
Test: atest KeyguardPinViewControllerTest
Flag: EXEMPT bugfix & test only fix
Change-Id: I9b50e46ed6fbee245faf444e79b2c2bfa49bb1da