Bluetooth 5: Update LE2M implementation through DM (1/3)

This patch moves LE2M implementation to the DM instead of tied to GATT.

Below are the reasons to make this change:
1) Per link setting usually be done under DM, such as link policy etc.
2) In the future, if LECOC is coming to use in more profile/apps,
   we can not rely on GATT API for these. Separate API should be required.
3) we could have multiple logical GATT user on top of the same link.
4) Also this will avoid duplicating all the APIs on GATTC and GATTS.

Bug: 37586939
Test: manual
Change-Id: I9e84b7b4aac1ccb7d3d1c4932d5da40784acf0d7