Floss: Refactor argument parsing in btclient

Refactor argument parsing so that:
* Always use safe indexing instead of unsafe `[]` (also fixes a case
  where it panics due to this).
* Do not hardcode argument length (bug prone).

Bug: 188718349
Tag: #floss
Test: Build btclient on Linux

Change-Id: I1aed068a014ac5cb3f6098a574b5e1ba8c0b5275