okio: increase segment size to 8 KiB
okio: increase segment size to 8 KiB

Original import was setting the segment size to 2 KiB with the comment:
  // TODO: Is 2 KiB a good default segment size?

Measurements shows that on encrypted devices download speed from
playstore is 3x faster using 8 KiB segment buffers.

Bug: 24253930
Change-Id: I25df01c9c4d1961757d3e1f5bb580965bb2bfdd4

Infinitive-OS/platform_external_okhttp,CyanogenMod/android_external_okhttp,Infinitive-OS/platform_external_okhttp,TeamExodus/external_okhttp,PAC-ROM/android_external_okhttp,geekboxzone/mmallow_external_okhttp,Infinitive-OS/platform_external_okhttp,AOSP-CAF/platform_external_okhttp,AOSP-CAF/platform_external_okhttp,CyanogenMod/android_external_okhttp,CyanogenMod/android_external_okhttp,boulzordev/android_external_okhttp,AOSPB/external_okhttp,TeamExodus/external_okhttp,SlimRoms/android_external_okhttp,AOSPB/external_okhttp,geekboxzone/mmallow_external_okhttp,PAC-ROM/android_external_okhttp,PAC-ROM/android_external_okhttp,geekboxzone/mmallow_external_okhttp,AOSP-CAF/platform_external_okhttp,AOSPB/external_okhttp,TeamExodus/external_okhttp,boulzordev/android_external_okhttp,boulzordev/android_external_okhttp