egltrace/android: Fix tracing Zygote processes

The root cause of the problem was a bug in the implementation of
getZygoteProcessName.  When the wrap.$procname approach is used, reading
/proc/cmdline produces "$procname\0/system/bin\0--application\0"... (with
embedded zero characters).

Fixed by simply not supplying length argument to the truncate call, which will
truncate to strlen().  The same bug is also present in getProcessName.
os: Improve String with more comments and checks.

Thanks to Alexander Monakov for drawing attention to inconsistencies in
os::String::truncate(void).
os: Ensure len is properly computed on all getProcessName code paths.

And that truncate is always called.

Thanks to Alexander Monakov for spotting this.