Revert "Make PlaybackState immutable with a builder"

Will resubmit tomorrow for Build Breakage Friday as this change needs new SDK prebuilts for unbundled app branches.

This reverts commit aa746b27610680a2c0fbdf7d81a6455c4597f05e.

Change-Id: I28ba7e1b09234a5eb1b725aed043b9de98e1bc16
7 files changed