pread() data into memory instead of lazily mmap everything am: b936892050 am: a949d23e78 am: 41f476a8ec am: 435179c9cc

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/2030887

Change-Id: I0f4d996e4a29cc1b48165983b94c16755c223399