audioflinger: fix offload resume after drain

When pausing and resuming during the drain,
the drain sequence number shuold not be modified
otherwise the drain callback will be ignored.
This causes failure to notify end of stream to audio
player and transition to next song.

Bug: 11247103.
Change-Id: I2a35c5cc3fd6aa667cdd257f9e9cc8715cef5159