Migrate the MediaSessions file to kotlin

This is in preparation for a refactor related to handling volume events
from system media routing sessions (aka mirroring sessions).

Bug: b/362507305
Test: Presubmit, build and verifying regular volume adjustment
Test: funcionality while casting.
Test: adb shell dumpsys activity service SystemUI | grep -A 100 "VolumeDialogControllerImpl state:"
Flag: EXEMPT Non-functional change
Change-Id: I5116a651c8710507b32eb4c25d1b47be8bc0ce86