- Get rid of TypeLiteral<T>. It was introduced to read the component
  type of a List<T> at runtime. But we use arrays everywhere else,
  and we can always read the component type of an array type at
  runtime.
- Properly read/write "minor" types (e.g. sint32, sfixed32). The old
  implementation could only read/write data as the "typical" types
  (one per Java type), e.g. java.lang.Integer -> int32, java.lang.Long
  -> int64. So if e.g. an extension specifies sfixed32 as the type, it
  would be read/written in the totally incompatible int32 format.
- Properly serialize repeated packed fields. The old implementation
  doesn't do packed serialization. As an added bonus, and to be more
  aligned with the rest of protobuf nano / main, repeated packable
  extensions can deserialize both packed and non-packed data.
- Split Extension class into a hierarchy so under typical usage a
  large chunk of code dealing with primitive type extensions can be
  removed by ProGuard.

Bug: https://code.google.com/p/android/issues/detail?id=62586
Change-Id: I0d692f35cc2a8ad3a5a1cb3ce001282b2356b041