Move Freezer flags to _boot namespace

Now Freezer "enable" flag belongs to _boot namespace but "timeout" is in
immediate one. This inconsistency confuses device_config. Solve it
by grouping them together.

Bug: 183713629
Test: device_config get activity_manager[_native_boot] freeze_debounce_timeout
Change-Id: I388215d5236b1f4450516acbe46fcb95457bf017