Handle negative background data usage

Due to periodic statistics recording, total foreground data bytes
may end up being more than total usage bytes for a bucket duration,
which leads to negative background data usage for a UID.

Modify recordUsage to update the data only if background usage
result is not negative.

Tests:
        Build, Boot
        Data usage related tests

Change-Id: Ia27a1d8e66b911b2137bb456d42c53da7a92dbb4
CRs-Fixed: 2571895