[fidl] [llcpp] Make fidl::StringView data const

FIDL-606 #done

Improves the ergonomics of fidl::StringView in llcpp.

Also hides the fact that it inherits from fidl_string_t, as casting it
to fidl_string_t would undermine const-safety, due to fidl_string_t
using a non-const data pointer.

Test: No behavior change; compiles.
Change-Id: Id12232962d4090b6b88ae689270e05e9dd926699