Add a backup manager constants secure settings.

This change add one secure settings which is a key value list consisting
of 7 individual settings; it also provides a class to get these settings
and use the default values specified in default.xml file when the settings
are missing.

This is the first part of a 2 part change. A subsequent change will use
these settings in backup service manger for setting up backup frequency.

Test: This change builds. Manual tests will be included in part 2 of the
change.
Bug: 63351792
Change-Id: I62baddbf3e0b96af6b7de3dfdeac3479e5b3ded8