Add Rust interface for DropBoxManager

Change Iddadeb5a175e395982ea632194f433536936aa26 refactored
DropBoxManager to not use a hand crafted Parcelable and thus be
compatible with interface. This change leverages aidl_interface to
create a Rust `DropBoxManager` client for use code bases that need one.

Bug: 420949170
Bug: 368152571
Test: atest DropBoxManagerTestRust
Flag: EXEMPT library only, no new functionality on device.
Change-Id: I9e07aee2311cddd7d5906989f730589139bdf449