fix: serializing whitespace and quoting priority

[IgorEisberg]

Not strictly useful for Apktool, this is just perfecting the YAML library.

• Fix leading/trailing whitespace not respected and properly quoted by the serializer.
• Use pyYAML's quoting priority as the standard: prefer single-quoting for literals, only fall back to double-quoting when escape sequences are involved.
• Rename escapeString/unescapeString to encodeString/decodeString, since they do full YAML encoding/decoding rather than deal with escape sequences only.

Added a few more test cases for the leading/trailing whitespace.