AAPT: Fix an issue where a resource name was incorrectly derived on Windows

Calling String8::getLeaf() will assume the system's file path separator,
however the source string was already converted to a unix path.
getLeaf() would therefore not find any occurence of '\' and would
return the full path.

Bug:18036805
Change-Id: Ic2bfac0cc553406740204a296327e266b05c0eff