Stop removing valid copy headers

My change to clean up obsolete copy headers would remove valid ones if
thhe LOCAL_COPY_HEADERS_TO path wasn't cleaned. I'm seeing this most
with values that just end in '/', so we end up with a '//' in the path,
which isn't textually equivalent, and we remove it.

Test: No longer seeing constant removals on internal products
Test: Set LOCAL_COPY_HEADERS_TO := ..
Test: Set LOCAL_COPY_HEADERS_TO := ../foo
Test: Set LOCAL_COPY_HEADERS_TO := /foo
Change-Id: Idbeeb207a2bb2a8da766473dbded877cec7c9cc1