Ask to enable touch exploration only the first time it enables the feature.

1. Now we are asking the user to grant permission to the service to enable
   touch exploration only the first time this service is enabled. If the
   service was uninstalled and then later installed we ask the user again.
   This avoids the scenario in which rebooting the device or upgrading an
   accessibility service leaves the device in a state in which the user
   cannot interact with.

bug:6582088

Change-Id: I51d24e4892b3b48c9fb11dfb09ec1118502ba526