Make a couple of constants in MediaSessionService configurable.

As part of exempting certain use cases from FGS-from-BG restrictions,
we added a couple of constants earlier to denote how long apps
receiving the media button callbacks should be allowed to start
FGS from BG. This change makes those constants configurable in case
we decide to increase or decrease the duration later.

Bug: 174699413
Test: manual
Change-Id: I2e59a73202dd9067098b653e728775eb3a8530f7