Add earlier CHECK in hidl_string

hidl_string needs to be zero-terminated so that
the kernel can make a copy with it and have it
safely copied over to the the other process (and
that process can use c_str safely).

Right now, this CHECK is in parceling code for
hidl_string, but moving it to setToExternal
and adding additional documentation for clarity.

Bug: N/A
Test: TH
Change-Id: I1e5f338baa5757ec541e0c54f89f64df0a9c9d61
2 files changed