libziparchive: ensure ReadAtOffset is atomic

ag/880725 modified ReadAtOffset to seek then read from the open
file descriptor. Previously pread64 was used to provide atomic
behaviour.

This causes races when multiple threads are trying to access data from
the file. This is supported, so this change reverts the relevant parts
of the above CL to restore the old behaviour.
Bug:27563413

Change-Id: I7bffd78da8c558745dfc3c072ba9691b1b15bb5b