Guard mUidBlockedState with a separate lock.

Instead of using the main 'mUidRulesFirstLock', add a
granular lock for mUidBlockedState so that network state
requests (which only use mUidBlockedState) doesn't get
blocked on the highly contended 'mUidRulesFirstLock'.

The risk with this change is that it is possible the
underlying firewall rules are not consistent with the
network blocked state that is returned to the apps.
But technically, this is already case if there is an
immediate update to network rules after app queries
it's network access state.

Bug: 209338078
Test: atest tests/cts/hostside/src/com/android/cts/net/HostsideRestrictBackgroundNetworkTests.java
Change-Id: I6a913742647577256c9281dd8ebf4aa5422a49ee