Fix 'Read-only filesystem' errors in Channels EncodingTest.

Merged tests relied upon files in the user_dir which could
produce the aforementioned errors, however, by switching the
creation to temp files we can resolve the issue.

Bug: 205565983
Test: just changes tests.
Change-Id: Ie1ff5b6b14c68e55c8637e81a7d9a5c8a0a38a56