Rename mTapPacketReader to mDowntreamReader am: 0bd10760a8 am: 7ea99be9d6

Original change: https://android-review.googlesource.com/c/platform/packages/modules/Connectivity/+/1824633

Change-Id: I57e3a7f336c9c740660bddcb1d6b5d35e58af793