binder: add an isHandlingTransaction method

This method is needed for aosp/1806578 because, although asynchronous
binder calls normally need to be offloaded to a separate thread, this is
not the case for such calls that happen during the handling of another
binder transaction.

The implementation of this method was based on the Java JNI binder
interface in `frameworks/base/core/jni/android_util_Binder.cpp`.

Test: m
Change-Id: Ia6d59f3f466911a27243476714cc5acf2eb4c629