[P2P] Eliminate separate storage of binders - a memory leak

A global storage of binders is replaced by attaching a binder
to its corresponding channel.

Bug: 62905652
Test: NonConcurrencyTest:test_*p2p* acts/sl4a tests passing
Test: p2p/aware behavior verified using Settings app
Change-Id: I08e8d8f5d56cfd81d078e74f3cf2ccc5d06f61e3