Add a sub-HAL implementation for testing multi-HAL

Creates a fake sub-HAL using the default implementation for sensors HAL
2.0 with some small modifications to support the multi-HAL interface.

This sub-HAL can be configured to support two different sets of sensors
making it easier to build and load two different sub-HAL implementations
onto device and verify the multihal implementation works.

Bug: 136511617
Test: compile only. Once multihal can load in sub-HALs, then this can be
accurately tested.

Change-Id: I9b136506bdbc8a3b196fd363748bddfcdd564daf