Rename isHandlingTransaction to isDirectlyHandlingTransaction

The change in aosp/1806594 is adding a method to the binder ndk called
isHandlingTransaction, but that method behaves differently from this
method when HIDL gets involved, as getCurrentServingCall returns
HWBINDER in that case.

Test: m
Change-Id: Ie2812943d1e98e99a1c361475965a2c75214724a