libbinder_ndk: set/getExtension

Similar to what is on IBinder. See comment on AIBinder_setExtension for
detailed information on usage.

This allows downstream codebases to tag on data to binders for their
extensions rather than having to modify the interface directly (which
breaks ABI compatibility and may cause more merge conflicts).

Bug: 136027762
Test: atest CtsNdkBinderTestCases
Change-Id: I2d1bb6cd906ff1974df8e81475d5fa8f569e9b6e