Fix the set member size check error

The default member device set object are not null. We should check it by
its size instead.

Bug: 205507889
Bug: 150670922
Bug: 178981521
Test: make -j50 RunSettingsLibRoboTests ROBOTEST_FILTER=CachedBluetoothDeviceManagerTest
Change-Id: Iae4b2f70723c1b083ad71df1374a608a12ff6c0e