[sunandapandit03]

Adds a handler for Android third_party METADATA files (protobuf text format),
as a first step for #2844.

What it parses:

• name and description
• third_party.version
• third_party.url entries: HOMEPAGE -> homepage_url, GIT -> vcs_url,
  ARCHIVE -> download_url
• last_upgrade_date -> extra_data
• license_type (RESTRICTED, BY_EXCEPTION_ONLY, ...) -> extra_data

Notes / questions:

• license_type values are Android license categories, not SPDX expressions,
  so I stored them in extra_data instead of extracted_license_statement.
  Happy to change this if you prefer a different mapping.
• Package type is "generic" for now.
• is_datafile skips *.dist-info and *.egg-info parent folders so PyPI METADATA
  files are not picked up, and requires "third_party" near the top of the file.
• Follow-ups in separate PRs: README.version and Android.bp.

Tests:

• New tests/packagedcode/test_android_metadata.py with two fixtures
  (exfatprogs and a BY_EXCEPTION_ONLY file).
• Existing tests/packagedcode/test_pypi.py still passes.
• Plugin listing fixtures (plugins_list_linux.txt, help.txt) updated by hand,
  because the list test only runs against the Linux file.