Add getClientTargetProperty API entry.

getClientTargetProperty will give hardware composer the ability to request some
properties of the client target that hardware composer wants. Prior to this
API, the client will does its best to produce the client target of which the
properties are pretty much fixed.

BUG: b/145968912
Test: mmma -j32 hardware/interfaces/graphics/composer/2.4/
Change-Id: I055f46b1eeba1d3e20d6a92a9d50a83e0f1ee694