ANDROID: binder: add min sched_policy to node.

This change adds flags to flat_binder_object.flags
to allow indicating a minimum scheduling policy for
the node. It also clarifies the valid value range
for the priority bits in the flags.

Internally, we use the priority map that the kernel
uses, e.g. [0..99] for real-time policies and [100..139]
for the SCHED_NORMAL/SCHED_BATCH policies.

Bug: 34461621
Bug: 37293077
Change-Id: I12438deecb53df432da18c6fc77460768ae726d2
Signed-off-by: Martijn Coenen <maco@google.com>