Add function to change LE Tx Data Length

As part of new SL4A tests for LE CoC to measure data throughput, this
commit adds a function to modify the LE Tx Data Length parameter to its
maximum.

Test: Ran the new ACTS Tests for LE CoC; BtCocTest and BtCoc2ConnTest
Bug: 70683224
Change-Id: I06d3f95a339dcdc310a18bcf17fbca8623f849d6