Fix parsing of shell command output regardless of encoding

We always know the start and end control patterns will be in UTF-8.
However, the content in between could be encoded differently.

Search for the patterns in the shell command output using raw bytes,
instead of a string pattern match.

Ref: QRDL-950
Change-Id: I6cb019b644d371c37593fe26784f6e9d41844dec
Signed-off-by: Roman Birg <roman@cyngn.com>