Migrate reference external HAL implementation to AIDL

Android T migrated the HAL interface to AIDL, but the reference HAL
was never updated to use AIDL. This CL migrates the reference
HAL implementation for external cameras to use AIDL as well. The
external HAL uses V4L2 standard to expose USB cameras to the
cameraserver.

The reference HAL implementation for internal camera was dropped
because it is not possible to write a generic HAL that works with some
large percentage of internal cameras.

Bug: 219974678
Test: Existing CTS tests pass with external camera connected.
Change-Id: I35f3dc32c16670eca7735a4ac00fed3daf36aa65