MAP: Handle concurrent MNS SDP Response and MAS Disconnection

Usecase:
1) Establish MAP Session without MNS Connection.
2) Remote sends MNS Registration and MAS Disconnect request
   in a very short interval (~100 ms)

Issue:
FATAL Exception in BluetoothMnsObexClient.

Reproducible Rate: One time issue.

Root Cause:
Once MNS SDP response is receved, active MAS connection is
disconnected which triggers stopObexServerSession. While
setMnsRecord() is processed in MAP Service Handler thread context,
main thread makes Handler thread of BluetoothMnsObexClient as null
which results in NullPointerException.

Fix:
Not to set Handler thread of BluetoothMnsObexClient as null.

Test: Sanity testing with BT on/offf and MAP usecases

Bug: 118602982
Change-Id: Id45896130f87ef5aef7278d260758efdeae89b52