Bitmap.cpp: replace dup() with fcntl(F_DUPFD_CLOEXEC)

Replace calls to dup() with fcntl(F_DUPFD_CLOEXEC). The only difference
between the two is that O_CLOEXEC is set on the newly duped file
descriptor. This helps address file descriptor leaks crossing an exec()
boundary.

Test: compiles and boots
Change-Id: I0c3b1baa49677a8e4831e1880e2d0ab38e08c6f4