OPP: Initialize OPP FileProvider after user unlock

* The FileProvider used by Bluetooth OPP profile used to be initialized
  before user unlock, resulting in file system access errors since
  credential encrypted volumes are not mounted and unlocked yet
* Incomplete initialization also leads to NPE when getUriForFile is
  called
* This change adds BluetoothOppProvider that only initializes after user
  unlocks the device

Bug: 36274847
Test: make, Opp regression test
Change-Id: I323b9084de1ff72922c4dd9866c36dd172696a8f