Calls setUidFirewallRules() and enableFirewallChain() asynchronously.

The updateRulesForRestrictPowerUL() method is used to set the
power-related restrictions for all apps and it ends up by calling
NetworkManagerService twice (and each call spawns an iptables process,
which takes ~50ms to complete). This method is called on some critical
paths, like when the device leaves Doze Light mode.

This change makes these calls asynchronously, hence reducing the delay
on such critical paths in the O(100ms).

BUG: 31281543

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

Change-Id: If720a7112c72a18193ea9614ae187b7ed9c741ec
(cherry picked from commit 84d3973d)