Fix MapClientTest#testConnect

The variable 'mMapInstanceMap' was not cleared
when service is stopped, which resulted testConnect fail
if a previous test added any key.

Bug: 264224883
Test: atest MapClientServiceTest#broadcastReceiver_withActionAclDisconnected_whenConnected_callsDisconnect \
             MapClientTest#testConnect
Change-Id: Ib9da20e0b00c23345b892f6eec680d2b44972349