Handle empty <cloud-backup> section in android:dataExtractionRules

Currently the <cloud-backup> section in android:dataExtractionRules is
ignored unless it contains rules. Instead, we should interpret it as
'everything other than cache and no-backup dirs is eligible for cloud
backup'.

Bug: 195095045
Test: 1. atest BackupEligibilityHostSideTest
      2. Use a test app with empty <cloud-backup> section to manually
      test:
          2.1. Empty section - everything is backed up
	  2.2. Empty section but "disableIfNoEncryptionCapabilitites"
	  set to "true" - data only backed up if the transport supports
	  encryption.
Change-Id: Ic8066721a46bda688f9211c51a0f2497e9caf93b