Changes to UNEXPECTED_NULL handling in libbinder_rs.

Two changes related to UNEXPECTED_NULL in libbinder_rs:
1) Return UNEXPECTED_NULL from Deserialize for SpIBinder instead of
   panicking
2) Convert UNEXPECTED_NULL status codes to NULL_POINTER exceptions,
   for compatibility with C++ and Java services.

Test: atest aidl_integration_test with 1406447 applied
Change-Id: I2e7cc92b2e1cf08a35968f675827d7d917144110