Expose timestamp source & thumbnail sizes characteristic

... for virtual camera.

Also fix .setAvailableCharacteristicKeys so it takes effect once
the .build() is called so it's not order dependent.

Bug: 301023410
Bug: 322965201
Test: atest android.hardware.cts.CameraGLTest
Test: atest android.hardware.camera2.cts.ImageReaderTest
Test: presubmit
Change-Id: I1fac9c4561c910f128fa67b7ce11d8e4c6c31b09