libbinder_ndk: define userdata class in headers

Rather than auto-generating it for each class.

From 'struct AIBinder' comment:
                         (process boundary)
                                 |
 MyInterface ---> AIBinder_Weak  |  ProxyForMyInterface
      ^                .         |          |
      |                .         |          |
      |                v         |          v
   UserData  <---   AIBinder   <-|-      AIBinder

ICInterfaceData represents 'UserData' here.

Bug: 127336441
Test: atest android.binder.cts
Change-Id: I2ffd7533dd81f06126547e4c82ec7e1e1cb0b0d0