AudioPolicyManager: fix DTMF gain over BT SCO

Similarly to voice communication, DTMF usage gain must be maxed out
when playing over Bluetooth SCO to avoid the double attenuation.

Test: repro steps in bug
Flag: EXEMPT bug fix
Bug: 367200697
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:c32e669e3da6ad44d86f9c486dfc301165731ebb)
Merged-In: I24f28c7f36c13f94fb68b44a4a5a0ccebaad7158
Change-Id: I24f28c7f36c13f94fb68b44a4a5a0ccebaad7158