Switch "adb sideload" over to lseek/read.

This made no difference to the time taken to sideload an AOSP marlin
update (1m24s), but does reduce the memory requirements to 64KiB.

Bug: https://code.google.com/p/android/issues/detail?id=258500
Bug: http://b/36046324
Test: "time adb sideload update.zip"
Change-Id: Ib0a4daeb5fe798a9bb9c9e0e5fbcff0ed46f20df