[Multi-user]: R backwards compatibility in activating/deactivating the service

Bug: 123347794

In Q, backup is OFF by default for non-system users. In R, we will change that to ON
unless backup was explicitly deactivated with a (permissioned) call to
setBackupServiceActive.
Therefore, remember this for use in R. Basically the default in R will be
rememberFile.exists() ? rememberFile.value() : ON
Note that this has to be done right after the permission checks and before any other
action since we need to remember that a permissioned call was made irrespective of
whether the call changes the state or not.

Test: 1. atest $(find frameworks/base/services/tests/servicestests/src/com/android/server/backup -name '\''*Test.java'\'')'
2. atest RunBackupFrameworksServicesRoboTests
3. atest CtsBackupTestCases CtsBackupHostTestCases GtsBackupTestCases GtsBackupHostTestCases

Change-Id: I0897d6f9f8d8d8a9fc3c0faabfc9045b416dd25f