Simpify logic in updateTcpBufferSizes

updateTcpBufferSizes() only need tcp buffer size as its
parameter. Also unify the logic to check default network
outside the function.

Bug: 120119769
Test: 1. Build pass.
      2. runtest frameworks-net

Change-Id: Iee9fec3efe7d5be5b590dd1c1f67ec5de636e613