Remove the old log files if cache space is insufficient for OTA

We set the limit of the max stash size to 80% of cache size. But the
cache space can still be insufficient for the update if the log files
occupy a large chunk of /cache. So remove the old logs for now to make
room for the update.

Bug: 77528881
Test: unit tests pass
Change-Id: Ia8bcb0ace11f8164ad9290bfb360e08e31d282cb