releasetools: Add two new entries to metadata.

Add "ota-required-cache" into the metadata file in an OTA package,
which shows the minimum free space on /cache to apply the update.

Add "ota-type" into the metadata file, which shows the OTA type for
this package (i.e. one of FILE, BLOCK and AB).

Also add the cache free space check into updater-script when generating
block-based incremental OTAs (we only had such lines for file-based
incrementals before).

Bug: 26731903
Change-Id: Id6ff0fc4cdfb1443636b0b3800b0f8bddb5bb1d0