Deinitialize rfcomm in the right order

btsock_rfc_cleanup() execution, namely cleanup_rfc_slot() may result
in remaining data being flushed when uid_set has already be set to
NULL, which will crash the bluetooth service. Move setting uid_set
to NULL to the very end of the btsock_rfc_cleanup() to avoid this.

Bug: 170775521
Bug: 199827901
Test: atest BluetoothInstrumentationTests
Test: IOP and BCST
Tag: #stability
Change-Id: Iaa9053ba179889b08375b904d14ff0421e84f93e
(cherry picked from commit 909e3813922055a90fdc6b32b7e5f5eb72691f2f)