logd: always compress SerializedLogChunk in FinishWriting()

When calculating the space used for pruning, if a log chunk is
compressed, that size is used otherwise the uncompressed size is
used.  This is intended to reach a steady state where 1/4 of the log
buffer is the uncompressed log chunk that is being written to and the
other 3/4 of the log buffer is compressed logs.

If we wait until there are no readers referencing the log chunk before
compressing it, we end up with 2 uncompressed logs (the one that was
just filled, that readers are still referencing, and the new one that
was allocated to fit the most recent log), which take up 1/2 of the
log buffer's allotted size and will thus cause prune to delete more
compressed logs than it should.

Instead, we should always compress the log chunks in FinishWriting()
such that the compressed size will always be used for log chunks other
than the one that is not actively written to.

Decompressed logs due to readers are ephemeral by their nature and
thus don't add to the log buffer size for pruning.

Test: observe that log buffers can be filled in the presence of a reader.
Change-Id: Ie21ccff032e41c4a0e51710cc435c5ab316563cb