frameworks/base: add "Accept all files" option for incoming files via Bluetooth [2/3]

This patch defines a key to store the option value in Settings Provider db.

Use together with https://gerrit.omnirom.org/6782/
 and https://gerrit.omnirom.org/6784/

Change-Id: I31275e3a88841891b9cd7c3109941821797cbab8