Ensure that local number is always a member of MMS groups

[dwong]

Fixes #7683

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• I am following the Code Style Guidelines
• I have tested my contribution on these devices:

• Samsung Galaxy S9, Android 8.0.0

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Description

New MMS groups do not initially have the local number as a member. As stated in the bug report #7683, this caused subsequent responses to create a new group since the membership is different.

Signal should behave similar for non-secure MMS groups as it does for Signal groups: the local number should always be a member of the group.

Tested this by:

1. Creating an MMS group from Signal
2. Receive response from another member
3. Verify that the group is properly threaded