Fix mutable bytearrays returned/used by some APIs.

Certain APIs and classes retain or return bytearrays
which can be mutated by the callers.

1. Use @Immutable annotations to statically check
   if a class exposes any mutable data.
2. Add an ImmutableByteArray class that prevents mutable
   bytearrays by making or returning a copy of an underlying
   set of bytes.

Change-Id: I2c5804eeb3d2e736eb45bb3283979b2874b5d7b4
ORIGINAL_AUTHOR=KB Sriram <kbsriram@google.com>
GitOrigin-RevId: 1c9804a2101360ddf41db719eef1d763e965d9ed