adb: add helper to consume a prefix on a string_view.

It's error-prone to manually writing code of the following form:

  if (foo.starts_with("some_prefix:")) {
    foo.remove_prefix(strlen("some_prefix:"));
  }

Add a helper to do that for us.

Test: mma
Change-Id: I5df391deba8b6c036fcbf17a1f1c79af8d9abd2b