apk/installed: parse Z: (file checksum) token in ParseInstalled

[Dev-X25874]

AddInstalledPackage writes a Z:<checksum> line for every regular
file that carries a checksum, and real installed databases are full of
them. ParseInstalled had no case "Z": handler, so those lines were
silently dropped on read-back — every reconstructed tar.Header came
back with a nil PAXRecords map and no checksum.

Add the missing case, mirroring the write path: allocate PAXRecords
on demand and store the value under paxRecordsChecksumKey, the same
key used by installRegularFile and AddInstalledPackage.