logd: update SerializedLogChunk for new prune calcuations

A previous change included sizeof(SerializedLogChunk) to the size of a
log chunk to more accurately track logd's log usage, but did not
update the tests that check this value, so this change updates them
appropriately.

Bug: 161179582
Test: logd-unit-tests
Change-Id: Ic37f07fff98c260dcf39b7cc79504c1c3fc2149d