libbinder: RPC reject excess threads

Existing code allows arbitrarily many threads to be attached to a
session, even though the server specifies a maximum.

Bug: 189955605
Test: binderRpcTest (it's not possible to exploit this with existing
    APIs)
Change-Id: I674f1cef759ae1c4fa7d0c27bbd8f8714f7c16ed
1 file changed