Add a shared header lib for libbinder/libbinder_ndk

The new header lib will be shared by libbinder and libbinder_ndk.
We'll put shared functionality here.
For now, it contains ToString() which is used by AIDL-generated
parcelables for both C++ and NDK backends.

Bug: 179020493
Test: aidl_integration_test
Change-Id: I930a334fa6d6f1a7b38729f8df6bcdb202c9f19c