Modify 5G RAT into Bearer field.

Only keep NR in here, remove GSM, TD_SCDMA, IWLAN and LTE_CA.

Bug: 143995814
Test: 1. Build pass.
      2. make RunSettingsRoboTests ROBOTEST_FILTER=ApnEditorTest
      3. check UI:
      3-1. Open APN settings page: Settings > Network & Internet > Mobile Network > Advanced > Access Point Names.
      3-2. Click the 3 dots at the up corner, and then select "New APN" > "Bearer"

Change-Id: I9b8efbe26c2702696f720f0f221f3d65f785e644