nuplayer2: Fix two use-after-frees

- std::ostringstream::c_str returns a string by value, so we can't
  immediately call c_str() on it and assign that to a variable.
- DrmUUID::arrayToHex returns a String8 by value. Similar "can't just
  call string() and assign that to a variable".

Both caught by our static analyzer.

Bug: None
Test: Ran the analyzer again

Change-Id: Ia9e0b2b00ee5150225dd487f3611a3893946fa4b