replace waitForBroadcastIdle with waitUntilBackupServiceIsRunning for system user

waitForBroadcastIdle was added to let GMSCore switch from D2DTransport (left over from
previos test) to GMSTransport on a boot complete broadcast. GMSCore
doesn't do that anymore so we dont need it anymore.

what we really need is is for the backupmanagerservice for system user
to come up to be able to use methods like userHasBackupTransport etc

also remove code duplication between this class and BackupUtils

Bug: 124035377
Bug: 137156961
Bug: 137197521

Test: atest -v CtsBackupTestCases CtsBackupHostTestCases
Change-Id: I987a20a936d3300830fa10acbfb22e43e481b1bd