MultiHal 2.0 - Get sensors list from subhals

Implement SubHal constructors. The default ctor will get the SubHal
object pointers from dynamic libraries. The test ctor will take a vector
of SubHal objects. Implment the HalProxy getSensorsList method which
will return all the sensors from the subhal objects. Create one unit
test for getSensorsList as well.

Bug: 136511617
Test: Flashed onto device and observed proper logs for both test
subhals. Also, passed getSensorsList test.

Change-Id: I6e8ca4883c50daabbd7b0955d3664f66b6cd74bf