Includes unit tests and covers zero-sized PSSHs, truncated PSSHs, and
various orderings of both v0 and v1 PSSHs.
Change-Id: I79ecc9475d483be610c003b57e831954a1c9800b
Most of these StringUtils were either operating on Uint8Arrays directly
or using strings to move around Uint8Array data anyhow.
Change-Id: I71efe6ede34210ca721d46acd53452344efff5bb