releasetools: Write back default_system_dev_certificate.

When signing a target_files.zip, the OTA certificate specified by
default_system_dev_certificate could be replaced with a mapped key. When
that happens, we must explicitly specify --package_key when generating
OTA packages with ota_from_target_files.py. Otherwise the OTA package
will be signed with the wrong key, which leads to verification failures.

This CL updates the default_system_dev_certificate value in
misc_info.txt accordingly.

Test: Sign a target_files.zip and replace the OTA key. Check
      META/misc_info.txt in the generated target_files.zip.
  $ ./build/make/tools/releasetools/sign_target_files_apks.py -v \
      --replace_ota_keys \
      -k build/target/product/security/testkey=build/target/product/security/platform \
      out/dist/aosp_marlin-target_files-eng.tbao.zip \
      signed-marlin-target_files-test.zip

Change-Id: I093234b5add3e27c5b3887cefeffd74e6f0a3e98