Warn more clearly when there are duplicate copied headers

When more than one makefile tries to copy a header to the same
destination, the warning is not clear, and hard to track down and assign
blame:

build/core/copy_headers.mk:15: warning: ignoring old commands for target `out/target/product/bullhead/obj/include/qcom/display/copybit.h'

With this change, the same behavior is kept, but the warning message is
more descriptive, and contains the offending Android.mk files:

build/core/Makefile:54: Duplicate header copy: out/target/product/bullhead/obj/include/qcom/display/copybit.h
build/core/Makefile:54: Defined in: hardware/qcom/display/msm8994/libcopybit/Android.mk hardware/qcom/display/msm8994/libcopybit/Android.mk

In this case, a $(CLEAR_VARS) is missing, so the same Android.mk file is
copying the same headers twice.

Bug: 27302058
Change-Id: Icf8f580ae71a78741db21c1d8f3213424459e637