Fix IME cannot show in lock task mode.

Replace check condition isKeyguardShowing instead of isKeyguardLocked in
shouldBeHiddenByKeyguard.
The check method isKeyguardLocked implies it will check if the device is
in restricted key input mode, which should be true for next time keyguard
shows. However, when keyguard is not showing, the IME should be visible.

Fix: 123399725
Test: atest KeyguardDisableHandlerTest KeyguardTests KeyguardLockedTests

Change-Id: I804920a44da33fef923ada8c05e08283e04b5aa0