Reduce libziparchive internal hashtable memory size

Debugging memory allocations on the nexus launcher unveiled significant memory allocations for the hashatable used in libziparchive, ~1MB.
This is partly because of the ZipString struct storing an entry in the table. The struct stored a pointer to a string (on 64 bit, 8 bytes) and the length to read from that pointer, 2 bytes. Because of alignment, the structure consumed 16 bytes, wasting 6 bytes.
Now, we store entries in the hashtable as a ZipStringOffset. This new structure stores a 4 byte offset from a fixed location in the memory mapped file instead of the entire address, consuming 8 bytes with alignment.

Bug: 79416399
Test: Builds successfully and manual testing by opening launcher on Pixel 2 shows precisely 50% decrease in memory allocated for the hashtable. From 909312 bytes to 454656.
Change-Id: I28b43699233fbee7f63fccae2d4fe96fcc07e5c4