NPMS: Cleaned up UID removal code.

When an UID is removed, NPMS needs to perform 2 tasks:

- Removed internal state associated with the UID.
- Call NMS to reset the iptables rules associated with the UID.

Currently, the UID removal logic is intermingled with code used used
during the UID state path, which has 3 drawbacks:

- Adds unnecessary comparisons.
- Makes the code more complicated.
- Is not handling all scenarios (like removing UID from fw_standby).

This CL simplifies how these 2 tasks are achieved.

Test: m -j32 cts && cts-tradefed run commandAndExit cts -m CtsHostsideNetworkTests -t com.android.cts.net.HostsideRestrictBackgroundNetworkTests

Change-Id: I12e9a6777b0d7168751571889fd7bb2d13934745
Fixes: 31377000