Move BinderProxy serialization into Java.

The BinderProxy class is not thread-safe, and all calls into
it were serialized by holding gProxyLock from JNI code. More
recently, we've been wanting to access BinderProxy from Java
code directly, and having the lock in native complicated things.

This change removes the lock in native code and adds it in the
Java layer. A benefit of this change is that it reduces the
scope of where a lock is held. On the flip side, we no longer
have a cached BinderProxyNativeData object lying around. This
means we now allocate/free a BinderProxyNativeData even if we
already have a Java object lying around for the native object,
which can happen quite frequently. But we deem the impact of
this to be acceptable.

Bug: 109888955
Test: sailfish builds, boots, proxy warnings still show
Change-Id: If2f4dbe5486ec7af0ef8ea42d24ac3a4330cc05a