Link Probing: Add feature flag

Link probing is disabled by default, enable it using
'adb shell settings put global wifi_link_probing_enabled 1'

Bug: 112029045
Test: atest SettingsBackupTest
Test: Manually check logs that link probing is not occurring when
disabled, and link probing occurs when enabled.

Change-Id: I43ceef59f5e3c76a229cb714e85d01ad16fe300e
diff --git a/core/tests/coretests/src/android/provider/SettingsBackupTest.java b/core/tests/coretests/src/android/provider/SettingsBackupTest.java
index 87ad3d1..a0387be 100644
--- a/core/tests/coretests/src/android/provider/SettingsBackupTest.java
+++ b/core/tests/coretests/src/android/provider/SettingsBackupTest.java
@@ -521,6 +521,7 @@
                     Settings.Global.WIFI_LINK_SPEED_METRICS_ENABLED,
                     Settings.Global.WIFI_PNO_FREQUENCY_CULLING_ENABLED,
                     Settings.Global.WIFI_PNO_RECENCY_SORTING_ENABLED,
+                    Settings.Global.WIFI_LINK_PROBING_ENABLED,
                     Settings.Global.WIFI_MAX_DHCP_RETRY_COUNT,
                     Settings.Global.WIFI_MOBILE_DATA_TRANSITION_WAKELOCK_TIMEOUT_MS,
                     Settings.Global.WIFI_NETWORK_SHOW_RSSI,