Add BackupManager#excludeKeysFromRestore()

Add the method as a hidden API for now. Move to System API later.

Bug: 145126096
Test: 1. Call BM#excludeKeyFromRestore for a test package
      2. Reboot the device
      3. Run restore for the test package
      4. Verify excluded keys are not restored

Change-Id: Ice57299eaf54cee8428b86e2d4f78de12b8db4fc