c2SoftAac: drain all samples from encoder at eos

at eos, the plugin is not waiting to collect all the samples that
are still with encoder. this is addressed.

Bug: 375055097
Test: atest MctsMediaV2TestCases:AudioEncoderTest
Test: atest MctsMediaV2TestCases:CodecEncoderTest
Test: atest MctsMediaEncoderTestCases:EncoderTest
Test: atest MctsMediaV2TestCases:CodecEncoderBlockModelTest
Test: atest MctsMediaV2TestCases:CodecEncoderMultiAccessUnitTest
Test: atest MctsMediaV2TestCases:CodecEncoderBlockModelMultiAccessUnitTest
Test: atest VtsHalMediaC2V1_0TargetAudioEncTest

Change-Id: I8352e9923c63e6fe73542fa13b80409e7d04bc2b