Handle Settings.Secure.backup_manager_constants=null correctly

CTS reads the value of Settings.Secure.backup_manager_constants setting
via a shell command, which means that if the actual value is |null|, it
gets converted to the string "null". Handle this correctly by checking
for "null", not literal |null|.

Bug: 63885845
Test: cts-tradefed run cts-dev --module CtsBackupHostTestCase
Change-Id: Icc0ff8c6ed185448fd78a7715bc413c8fd381c64
1 file changed