Delete Bluetooth device cache regularly (#2751)

* Delete Bluetooth device cache regularly

Delete stale Bluetooth devices from the BlueZ device cache every week.
This makes sure that the overlay partition doesn't run out of inodes
which has happened in real world scenarios where many new Bluetooth
devices are discovered.

BlueZ maintains these files on a best effort base. So removing them
while BlueZ is running should be safe.

An alternative considered  was to lower BlueZ GATT caching (e.g. by
using Cache=yes instead of always, to cache only paired devices).
However, this would hurt performance and battery lifetime of Bluetooth
devices due to additional unnecessary GATT attributes reads. This is in
particular true for Bluetooth 5.1 devices which support the Database
Hash charactristic. Caching has also helped reliability with
intermittent connections (see
https://github.com/bluez/bluez/issues/191).

More importantly, besides the GATT attribute cache the same files are
also used to cache the device names as well. This is independent of the
above mentioned GATT cache configuration (see device_store_cached_name
in BlueZ). So disabling the GATT caching alone wouldn't solve the
particular problem we are facing.

See also: https://github.com/home-assistant/supervisor/issues/4490

* Use access timestamp instead of modification timestamp

The modification timestamp gets updated regularly (on each connect) it
seems. However, using access timestamp might be more accurate, as it
seems to preserves slightly more cache files. This additional devices
might be devices we don't regularly connect but are still around (and
therefor we shouldn't reread the GATT attributes regularly).

So deleting cache entries with access time older than 7 days. Which
essentially deletes all the entries of devices which haven't been seen
the last 7 days.

buildroot-external/rootfs-overlay/usr/lib/systemd/system/haos-bt-cache.service

@@ -0,0 +1,15 @@
+[Unit]
+Description=Remove Bluetooth cache entries older than one week
+
+[Service]
+Type=oneshot
+ExecStart=/bin/sh -c 'find /var/lib/bluetooth/*/cache -mindepth 1 -type f -atime +7  -exec rm {} \;'
+PrivateDevices=yes
+PrivateNetwork=yes
+PrivateUsers=no
+ProtectKernelTunables=yes
+ProtectKernelModules=yes
+ProtectControlGroups=yes
+MemoryDenyWriteExecute=yes
+SystemCallFilter=@default @file-system @basic-io @system-service
+

buildroot-external/rootfs-overlay/usr/lib/systemd/system/haos-bt-cache.timer

@@ -0,0 +1,13 @@
+[Unit]
+Description=Remove Bluetooth cache entries
+ConditionPathExistsGlob=/var/lib/bluetooth/*/cache
+
+[Timer]
+OnCalendar=weekly
+AccuracySec=1h
+Persistent=true
+RandomizedDelaySec=6000
+
+[Install]
+WantedBy=timers.target
+