Improve logs for unregistering routers and managers

After this change, MR2SI logs whether an unregistration was triggered
explicitly or by a process dying. Also, MR2SI now logs when there is an
attempt to unregister an unknown manager, instead of failing silently.

Test: adb shell am crash <app with router/manager>
Bug: 205124386
Change-Id: Ib7f22d080061b1d25e17cb92aad588350e6a094f