Remove DropBoxManager::getNextEntry

- Remove DropBoxManager::getNextEntry as the method was unused.
- Stop DropBoxManager::Entry inheriting from RefBase.
  This was causing bugs since DropBoxManager constructs Entry
  on the stack (in a non-ref counted context).

Bug: 119655216
Test: make
Change-Id: If0852ab6bcc1a92560132628897977eac16fddea
2 files changed