Make IDropBoxManagerService compatible with aidl_interface

IDropBoxManagerService referenced hand-crafted parcelable
DropBoxManager.Entry, making it incompatible with aidl_interface.
This change makes the AIDL files compatible by wrapping the internal
implementation:

- Removes `DropBoxManager.aidl`, which referenced a C header to include
  the manually implemented Parcelable `DropBoxManager.Entry`.
- Moves `Entry` to an AIDL parcelable within
  `IDropbBoxManagerService.aidl`
- Modifies `IDropBoxManagerService.java` to return
  `IDropBoxManagerService.Entry` instead of `DropBoxManager.Entry`, in
  accordance with the new AIDL surface.
- `DropBoxManager.h` implemented `DropBoxManager.Entry`, but it wasn't
  used anywhere in native code. Removes the `Entry` implementation
  there.
- `DropBoxManager.java` implements `DropBoxManager.Entry` as part of its
  public API surface. Thus, we can't remove this manually implemented
  Parcelable. Modifies `DropBoxManager.java` to wrap the new
  `IDropBoxManagerService.Entry` returned by the service and return
  a `DropBoxManager.Entry` constructed from
  `IDropBoxManagerService.Entry` where necessary. This keeps the API
  surface the same.
- Adds `DropBoxManagerTestCpp` as a sanity check to ensure the native
  API still works.

Bug: 420949170
Bug: 368152571
Test: atest DropBoxManagerTestCpp
Test: atest CtsDropBoxManagerTestCases
Flag: EXEMPT pure refactor, no API or behavioral changes
Change-Id: Iddadeb5a175e395982ea632194f433536936aa26