mediaswcodec: Set "default" as preferred service

Usage mapper will be retrieved from the preferred service if system
properties for mediaswcodec's ION usage are not set.

Test: Define system property "ro.com.android.media.swcodec.ion.heapmask"
and check the log during the startup of mediaswcodec.

Test: Manually log the mapper activity; test if gets invoked from
mediaswcodec and CCodec.

Test: atest CtsMediaTestCases -- \
--module-arg CtsMediaTestCases:size:small

Bug: 144995284
Change-Id: Iebcd89bfa38de3c10b79f878462ba2af347ad332
Merged-In: Iebcd89bfa38de3c10b79f878462ba2af347ad332