Fix cleanup of dead GATT clients

A bug was introduced in commit
307fe304445c0e36af510bdb58d3ae667a1f0dfe which causes normal
GATT clients (neither scanners nor advertisers) not being freed
properly when remote process terminates, leading to out of space
of available GATT clients after some time.

This patch adds back a ClientDeathRecipient to handle this.

Test: kill an app that has gatt client, restart it and create new
gatt client, verify the client id is same as before in logcat

Change-Id: I318cb1693984444a172cb81be2023a064dbc02a4