Create a key value settings observer for backup parameters

Extracts an abstract class to observe changes in backup parameter settings that
are stored as a comma-separated key value list. This class is
responsible for registering and unregistering a content observer on the
setting and updating local references to the parameters.

Refactor BackupManagerConstants and LocalTransportParameters to use this
implementation. This will also be used for the new backup timeout
setting.

Bug: 74346317
Test: 1) m -j RunFrameworksServicesRoboTests ROBOTEST_FILTER=BackupManagerConstantsTest
2) gts-tradefed run commandAndExit gts-dev -m GtsBackupHostTestCases -t com.google.android.gts.backup.TransportFlagsHostSideTest

Change-Id: Id4c50fbcf7479c925515887e3fa70e166dd9955c
(cherry picked from commit 16ef6ffb3cbea77637ad99c2b0c79cbfd8f03c9a)