Reconcile binder JNI with earlier changes, removing lock.

Change Idbacb6ad14c43aff8030d70b5e17427b86e92d6e introduced
synchronization of the sProxyMap in Java, removing the need
for serialization (and having a native lock). But for some
reason, we did not remove it in AOSP.

Bug: 132922399
Test: builds
Change-Id: Ib2dc64a7716a1e4798b998e54fac298eb40a7c1d
Merged-In: I4e06b3f93e30ed1c7868ec9e018709a7e796e441