AudioAttributes: fix setter for flags

A regression was introduced by d0c49687 which prevented @SystemApi
flags to be set in the builder of an AudioAttributes, causing
FLAG_BYPASS_INTERRUPTION_POLICY and FLAG_BYPASS_MUTE to get
ignored.
Note that even though any client can set System flags, they will
be discarded and ignored server side in the audio policy service if
the caller doesn't have the permissions.

Bug: 154205921
Test: fire alerts with packages/apps/CellBroadcastReceiver/tests/testapp
Change-Id: I9d593787c18a510aa702dc0da85254f0d9574f11