Avoid oem-unlock preference controller crash

Openning developer options will crash if the device doesn't support
oem_unlock module. We added a condition to check that the
oem_unlock_supported property is existing before getting OemLockManager.

Fixes: 136108893
Test: robotests
Test: lunch aosp_cf_x86_phone-eng; m dist; acloud create --local-image
Test: lunch sdk_gphone_x86-eng; make; emulator
Change-Id: I2c1821fbdd18e3e8162a492e1e15dd34c8aec803