media: further miscellaneous fixes to VT contribution

- remove unnecessary shared library dependencies
  (this frees up about 2MB runtime space)
- ASessionDescription: pull out default AS as a constant
- ASessionDescription: populate frame size only for video session

Bug: 153907802
Bug: 121230209
Test: build
Change-Id: I29d38b96d794615f4aead2bb98d147831070d25c