Add logging for error conditions when reading or writing backup enabled

state.

Also add debug logging when reading or writing backup enabled state to
be able to better investigate bugs.

Bug: 148587496
Bug: 147352819

Test: atest -v RunBackupFrameworksServicesRoboTests
Test: atest -v $(find frameworks/base/services/tests/servicestests/src/com/android/server/backup -name '\''*Test.java'\'')'
Change-Id: I3c9b158ce57558daa5437cebe6aa0a0c924692fc