Add system feature flag FEATURE_WIFI_PASSPOINT

This feature will be used to indicate device's support for Passpoint.
The apps will be able to call PackageManager#hasSystemFeature(FEATURE_WIFI_PASSPOINT)
if Passpoint is supported or not.

While there, remove the legacy build config flag for enabling Passpoint
(aka Hotspot 2.0).

Bug: 36886243
Test: Manual test
Change-Id: I99e9299a1ddbb8024f12a5a90a975eb54246cc97