audio: Reduce minimum offload fragment size for direct pcm

Minimum offload fragment size is huge and this results in
larger buffer being passed from AudioFlinger to HAL. As
shared block in AudioFlinger is also of the same size,
track is going into under run state multiple times at the
beginning of the playback. This is causing A/V Sync issues
during video playback.

Fix is to reduce it to 512 to be inline with other targets
Fixed alignment code as well

Change-Id: Ifc667a5f0ccf8f36c44d04c270740ed67f2a32b7