AttestedKeyPair: Address API review comments

Make AttestedKeyPair c'tor accept a List<Certificate> rather than
Certificate[] to match the getter method on this class.

To make it easier to use this class from other framework code I've
re-instantiated the c'tor with a certificate array which will
convert the array to a list.

Bug: 139092002
Test: cts-tradefed run commandAndExit cts-dev -m CtsDevicePolicyManagerTestCases -t  com.android.cts.devicepolicy.MixedDeviceOwnerTest#testKeyManagement
Change-Id: Ie80dcb28f112efa89d3cc6fdceb1b9e5e26c58b1