Add a new feature flag in android.net.wifi

Add a new feature flag - "network_provider_battery_charging_status" in
android.net.wifi package.
It is the same flag which was added in ag/25092320 in the wifi module.

framework-minus-apex depends on a flag defined in wifi_aconfig_flags
but framework-minus-apex does not depend on the corresponding aconfig
java lib wifi_aconfig_flags_lib. Therefore Soong doesn't know about
the dependency -> Soong won't tell Metalava about the flag ->
Metalava will default to reverting the API to the previous SDK
snapshot -> API will not be present in the SDK and docs artifacts.

Solution:
1) define a new flag in frameworks/base
2) Work with relteam to ramp it to next ASAP
3) Move NetworkProviderInfo to use this new flag

Bug: 339181644
Test: N/A
Change-Id: I358638546dc888a41498ec066d548c6612ea4f60

wifi/wifi.aconfig

@@ -9,3 +9,11 @@ flag {
     bug: "313038031"
     is_fixed_read_only: true
 }
+
+flag {
+    name: "network_provider_battery_charging_status"
+    is_exported: true
+    namespace: "wifi"
+    description: "Control the API that allows setting / reading the NetworkProviderInfo's battery charging status"
+    bug: "305067231"
+}