Implement GATT service cache

When trying to reconnect a previously paired device, notifications may
be sent by the remote device before the characteristic handles have been
discovered. Thus notifications will be lost.

This patch adds a non-volatile attribute cache that allows attribute
handles to be restored before notifications are received.

Bug: 17999991
Change-Id: I97faefbc6a2fed86cbce7f64d620ed03944d89b2