Partial revert of keyguard checks

startPreHideAnimation could be called even when the keyguard isn't
showing. This leads to some inconsistent state if we don't call
mBouncer.startPreHideAnimation.

Bug: 155706342
Test: manual (log startPreHideAnimation calls)
Change-Id: I039548a0e9a623a5e478e94490d3b695ae11d15c