Fix multiple copies of read-only files in sbox

Sbox preserves the permissions of input files when copying them into the
sandbox.  A read-only file copied into the sandbox multiple times causes
a permission denied error on the second write.  Building in Bazel results
in more read-only files, which triggers the issue on existing sbox rules
with duplicate input files.  Remove the destination file when copying if
it exists.

Bug: 184113103
Test: m USE_BAZEL=true
Change-Id: I7edf92d82b766100e3cbbd90d22428269d7d0167