[zip] Optimize mmap-based zip reader for zero copy

Change the interface of the MappedZipFile to return the original
mmapped buffer insted of copying the data over when possible.

+ some small optimization of the internal classes for memory
  usage
+ get rid of unneeded virtual dtors in Reader and Writer
  interfaces

Bug: 237583012
Test: Unit tests + boot
Change-Id: I3f83f2db58e3f80facd3185f723c3a158952c071
8 files changed