ANDROID: binder: fold common setup of node_prio

The setup of node_prio is always the same, so just fold this logic into
binder_transaction_priority() to avoid duplication. Let's pass the node
reference instead, which also gives access to node->inherit_rt.

There is no functional impact from this patch.

Bug: 148101660
Signed-off-by: Carlos Llamas <cmllamas@google.com>
Change-Id: Ib390204556e69c4bc8492cd9cd873773f9cdce42
(cherry picked from commit 498bf715b77c68e54d0289fa66e3f112278f87dc)
[cmllamas: fixed minor merge conflicts]