Cache camera id in VirtualCamera instance and make it immutable.

... otherwise getId returns 0 after camera unregistration which is
bit confusing, since one would expect the id is immutable during whole
lifetime of the VirtualCamera instance.

Bug: 310857519
Test: atest VirtualCameraTest
Change-Id: I432e0a69389af3d59a1a22b469b689581d424fb2