Fix test failure in MixedDeviceOwnerTest#testSecondaryLockscreen.

We assumed that the KeyguardSecurityContainer could only have one child;
that assumption was incorrect. Updated code to (1) explicitly search for
KeyguardSecurityViewFlipper when it is required, and (2) measure
multiple children in onMeasure.

Test: Verified with atest
CtsDevicePolicyManagerTestCases: com.android.cts.devicepolicy.MixedDeviceOwnerTest#testSecondaryLockscreen
Bug: 179313275
Change-Id: Iaec57d70c33149e0a2a6867efd9c9ba09d9d6946