Fix reliable write.

We cannot simply assume the write is terminated in reliable write. When
the reliable write value is longer than MTU allows, the current
implementation can only send whatever MTU allows and naively set the
status to GATT_SUCCESS, in the name of "application should verify handle
offset and value are matched or not". That's why MTU negotiation is a
workaround as people mention in b/37031096, which just fits all the write
value into a single request.

This also blocks our test on CtsVerifier.

Bug: 37031096
Test: Manual test and confirm that we don't simply send partial value
Change-Id: I907877608f4672f24c002e630e58bf9133937a5e