Address issues with ancestral serial number file

There were a few issues with the ancestral file storage methods which
I've fixed in this refactoring;

1) The log message for "get" said it was doing a write
2) The logic to get the file created a new file even if this was a read
3) The reading of the file asked for write permissions which it didn't
need
4) By asking for the correct permissions we can use the RandomAccessFile
constructor behaviour to create a file if needed.
5) By asking for the correct permissions we can catch the FileNotFound
exception thrown when the file doesn't exist and avoid creating an empty
unused file

Fixes: 157922462
Test: atest UserBackupManagerServiceTest ProfileSerialNumberHostSideTest
Change-Id: Ia5b070d2dbe9a778ec39473ec484f889ca8a8214