Rename mTapPacketReader to mDowntreamReader am: 0bd10760a8 am: 7ea99be9d6 am: 4a96283c13

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

Change-Id: Ia94f0340e86c93b1d4c7330206df424428f53f83