MainActivity Code Inspection and cleanup

Remove warnings from Code Inspection.
Collapse calls A->B->C to A->B.
Use onPause/onResume to resume and pause the Scan Service.
Merge branch 'master' of https://github.com/google/physical-web into MainActivity-Cleanup

Conflicts:

android/PhysicalWeb/app/src/main/java/physical_web/org/physicalweb/MainA
ctivity.java
Merge pull request #71 from schilit/MainActivity-Cleanup

MainActivity Code Inspection and cleanup.
Looks good to me. Thanks!
Merging.