Add TapPacketReader#startAsyncForTest, poll am: 046d253d63 am: 97825354b8 am: 82094abeb0

Original change: https://android-review.googlesource.com/c/platform/frameworks/libs/net/+/1435132

Change-Id: Ib1506e83fb3f8ac5f5d1e1d54b98583cd71b202e