Add a class for retrieving server flag values

Also adding two flags for previously hardcoded timeout values to test
out the new flag system.

The reason for adding a util class is to avoid calling DeviceConfig and
passing in the same strings every time a flag value is retrieved.

Bug: 210083465
Test: atest BackupAndRestoreFeatureFlagsTest.java
  Manual:
    adb shell device_config put backup_and_restore backup_transport_callback_timeout_millis 1
      Observe that transport status callbacks fail with a timeout error
    adb shell device_config put backup_and_restore backup_transport_future_timeout_millis 1
      Observe that many transport futures timeout
  Note that the timeouts that turned into flags are not unit-testable since
  they use the system clock to keep track of time. AFAIU this can't be
  mocked.
Change-Id: I6953fd480ec217a854a57e3c369cadb0947bd55c