Refactor RFCOMM / BluetoothSocket usage of LocalSocket

LocalSocket is used by BluetoothSocket. BluetoothSocket
passes a pre-created file descriptor to LocalSocket that
is then given to the LocalSocketImpl.

Commit b08c7bc0 broke the
behavior. Commit 7a8c36aa
put in a minimal fix.

This change tidies up LocalSocket and associated classes
and replaces a specialist constructor with a factory method
to highlight the special case. While there an unnecessary
exception has been removed.

Bug: 34111534
Test: Boot device
Test: vogar --mode app_process tests/tests/net/src/android/net/cts/LocalSocketTest.java
Change-Id: I4ba2f2d9ea361a950ff8bc8d64fc800d998c3210