Delay the write-back of misc_info in sign_target_files_apks.py.

Currently we're writing META/misc_info.txt to the new TF.zip during
ReplaceVerityPrivateKey(). We should delay that until we have replaced
everything in need. Otherwise we won't be able to replace/overwrite
that zip entry (unless `zip -d` first).

This CL also cleans up the return value of ReplaceVerityPublicKey() and
ReplaceVerityKeyId(), since the caller no longer needs the values.

Test: sign_target_files_apks.py and check the generated signed TF.zip.
Change-Id: I9fbd7182247728281519e5e3971557f6b018ad65