Floss: disconnect from DBus in MMC deconstructor

There is a connection leak in the constructor. This CL does the
appropriate cleanup in the corresponding deconstructor.

Bug: 309382806
Tag: #floss
Test: atest net_test_stack_btm
Test: do profile-switch 257 times
Change-Id: I5c909d252fb5762abbd096dcc4496ff980b327b5