Fix for DropBoxManagerService on devices with large storage

DropBoxManagerService does not save any data to the storage
due to int overflow when it's trying to get available space
on devices with large storage (like 1TB)

Bug: b/131354250
Test: Run CTS with command "run cts -m CtsIncidentHostTestCases
	-t com.android.server.cts.ErrorsTest"

Change-Id: Ib1b0d4decb48e9e771ef39127364131e6aa05662
Signed-off-by: Dmytro Chystiakov <dmytro.chystiakov@intel.com>