bluetooth_msm: This is a combination of 3 commits.

Bluetooth: Avoid window leak on device preference orientation change

Handle proper clean up upon orientation change to avoid window leak.

Change-Id: Iab9de97c4fc0a3bbb8bf2255c39748824b8338ea
CRs-fixed: 415979

Bluetooth: Convert AlertDialog to DialogFragment to avoid window leak.

Convert AlertDialog that prompt receiving reject or incorrect PIN
while BT pairing to DialogFragment that handles window leak on
configuration changes like orientation change.

Change-Id: If948ab74b1b2378fee6feb9fdb194303ab93d7fc
CRs-fixed: 472148

Bluetooth: Added null checks for HeadsetService.

This change avoids settings app crash due
to BluetoothHeadsetService being null.

CRs-Fixed: 506024
Change-Id: I81ec11e194dd6e2eb2a416e77dcebdef669c6914