Fix double mutex unlock and if() condition

Commit "L2CAP and SDP Search API for BT profiles (2/2)" introduced two
separate issues in btsock_rfc_signaled():

1. The "slot_lock" potentially gets unlocked twice, causing
   undefined behaviour.

2. The call to BTA_JvRfcommWrite() was incorrectly taken outside
   of the conditional if() block that only executes the code if
   data is available.

This patch reverts the relevant locking change and fixes the if()
condition.

Bug: 27204458
Change-Id: I63b1257d21ca7bdd248858ec67e28f64e37a7895