Fix rounding error accumulation of audio timestamp

The calculating method of audio timestamp would cause rounding error
accumulated with a long time recording, as each current timestamp is
calculated with the previous timestamp.
This patch changes to calculate all timestamps on the basis of the
first frame to avoid accumulation of rounding error.

Bug: 128277968
Test: CTS - MediaRecorderTest
 ANDROID_BUILD_droid-cts/tools/cts-tradefed run cts -m
 CtsMediaTestCases --test android.media.cts.MediaRecorderTest

Change-Id: I3464d62c30e7b1fd0976419b845e6b9067f44615