[non-BE] Resolve "JBM and DTX interaction not as expected"

• Related issues: #1122 (closed)
• Requested reviewers:

Reason why this change is needed

• Decoding a bitstream with DTX in VoIP mode with the neutral delay profile is not BE to non-VoIP mode.
• More specifically, in DTX, the decoder played out SID frames too early in VoIP mode compared to non-VoIP mode.

Description of the change

• Fixed JBM logic not to set target playout delay to 0 during DTX. Instead, the safety margin set from outside (60 ms in our case) is adhered to.
• Replaced occurrences of safety margin hard-coded to 60 ms with the value stored in JBM state.

Affected operating points

• Decoder output is non-BE only when decoding a bitstream with DTX in VoIP mode.

Related to #1122 (closed)