le_audio: Set default value for allowed context mask

Follow up fix CL for aosp/3049694.
We need to set default value for Integer allowed context mask to avoid
NPE.
Those value usages are behind feature flag.

Tag: #Bug
Bug: 331682466
Bug: 336468573
Test: atest LeAudioServiceTest BassClientServiceTest
Change-Id: I937ee65fa7102dee619a3f583912ace9625660ae