Add support for RAT-based TCP buffer sizes

If the RAT-based resource overlay is not set,
config_tcp_buffers will be used instead.

Test: atest FrameworksTelephonyTests
Bug: 222323543
Change-Id: I55ccb2a2f0b2408e97aa851b5a6e77cf2abac9f2
Merged-In: I55ccb2a2f0b2408e97aa851b5a6e77cf2abac9f2