floss: Fix disconnect order and gatt cache

Disconnect was removing the client before trying to access that client
id to send callbacks (hence we were never seeing gatt disconnect
callbacks). Also updates the gatt cache directory to one that is
accessible on ChromeOS/Linux.

Bug: 247605554
Tag: #floss
Test: Manual test on ChromeOS
Change-Id: I932e2ae623b6ef572d18b7af9885799a3ec44810