NPE in BluetoothPhoneService.java when switch users.
NPE in BluetoothPhoneService.java when switch users.

The null pointer is the mBluetoothHeadset is reset to NULL
when the  the headset profile is disconnected during user switch.
The fix should be  to check the null pointer before access mBluetoothHeadset
also fix all the potential NPE problems not just this one happened.

bug:11804919
Change-Id: I2acd0092179fa07293f9fcc4c9b65f1c3343a37c
(cherry picked from commit b19f55ba5a568a7399d8dd982007264287f6470c)

md5555/android_packages_services_Telephony