Add additional context to MtpDatabase

MtpService has been changed to a singleton service
running in user 0, so the original context will
be that of user 0. Accesses to MediaProvider and
MediaScanner should use the new passed in user context
in order to access user data, while the broadcast
receiver for battery data will use the original context.

Bug: 64822515
Test: Use Mtp and switch users.
Change-Id: Ic181258337944db7dc6f6097c967cb104df938d3