Revert "Make MtpReceiver and MtpService single user."

In P the system user can no longer access files of other
users. Now that descriptors are owned by UsbService, we
can revert to having multiple MtpServices without having
synchronization problems.

This reverts commit 08bae5c84fd7f2e3f876e60082b37e2865f5ab8d.

Bug: 72877143
Test: Use Mtp from multiple users

Change-Id: Ida57416ea3379758f3c67bb4acc6cab4d71b3deb
3 files changed