libbinder: replace RpcTransport::peek() with pollRead()

peek uses MSG_PEEK internally which only works with sockets.
This refactors that function into a more specific pollRead()
which works for e.g. pipes and other files.

Bug: 224644083
Test: atest binderRpcTest
Change-Id: I340fa7ee7a7bb2077115fc28835bcadf67db03d9