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