libsnapshot: Cache Ops correctly

Change the meaning of batch_size_. Previously, a batch size of 200 meant
200 compressed data ops. With variable block size, each compressed data
op can be up to 256k uncompressed size -> batch size meaning should be
changed to 200 blocks of block size.

With this being said, the default batch size can be increased to 4mb to
better accomodate variable block size

The way we calculate the number of blocks to compress at once also
needs to be changed. Since there's no way of determining the comperssed
data size ahead of time, allow overwriting the cache by batch_size_ and
then flushing the cache as needed

Bug: 322279333
Test: Ota on pixel and measuring system image cow
Change-Id: Ie8e08d109dc5c3b4f5f36a740bbbcd37362a7ab3