Expose functions used to add entries to the zip file.

Currently only dumpstate.cpp is using the new functions and variables
used to add an entry to the zip file so they're defined locally, but
they should be exposed through the .h so they can be used by the
device-specific dumpstate_board() implementations.

This CL moves the following definitions to the .h:

- bugreport_dir
- ZIP_ROOT_DIR
- add_zip_entry()
- add_zip_entry_from_fd()

It also refactored functions that were taking a specific file location
to use bugreport_dir instead, to avoid pollution on main().

BUG: 28867848
Change-Id: Ic5faa203b0863bc950d640f22fb91977e52b6bfc