AudioFlinger: lower FAST denied warning level

Changed ALOGW to ALOGD because the warning was causing
people to think a bug had occured.

Bug: 139825832
Test: Enter:  adb logcat | grep "denied by server"
Test: Launch OboeTester
Test: Tap TEST OUTPUT
Test: select SRate: 32000
Test: Tap OPEN
Test: logcat should show "AUDIO_OUTPUT_FLAG_FAST denied by server"
Test: Should be level D instead of W
Change-Id: I1f3f231d4304a1132964fbb1efce5922e7b91ae7