[floss] Improve heuristics for whether a dual device uses LE or classic

When displaying the list of devices to pair, we filter classic and LE
devices differently. Dual device is problematic because it's difficult
to determine whether we should filter them the classic or the LE way.

Currently, we just treat dual devices without the class as
non-connectable. Problem arises when those devices advertise the class,
but is non-connectable, because it will show up in the list of pairable
devices, but it shouldn't.

This CL adds another heuristics: Random address must indicate an LE
device. It also factors out the heuristics function.

BUG=b:312859619
TEST=Verify with the buds in the bug. It should only be in the pairing
     list only when it's put in pairing mode.

Change-Id: Ib41dc1d59018edca730964bb5059cc01fbdbb0c5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5189864
Reviewed-by: Gordon Seto <gordonseto@google.com>
Reviewed-by: Hsin-chen Chuang <chharry@google.com>
Commit-Queue: Archie Pusaka <apusaka@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1251306}

• device/bluetooth/chromeos/bluetooth_utils.cc[diff]
• device/bluetooth/chromeos/bluetooth_utils_unittest.cc[diff]