releasetools: Properly handle custom values for tools

Rather than writing these values to their own files, store them
in the target_files META/misc_info.txt. This way, we don't need to
download and parse through multiple files when using signing/release
tools.

Change-Id: I6f845fc055e4ef2ed4adf9628cca505e99b2a9fa
(cherry picked from commit d8b439ae)