Set sJniAdapterServiceObj to NULL after deleting it.

The global reference of sJniAdapterServiceObj(also other Objs)
object was deleted,  but we does not set it to NULL. As a result,
crash happened later when we access this object again.

Test: Enabled multi users and turn on/off bluetooth in multi
user spaces over 1000 times, everything is okay.

Bug: 38485697

Change-Id: Idb61b21b97bbe56c02bcf255c195cc33d5514ed1
Signed-off-by: xutianguo <xutianguo@xiaomi.com>