[Passpoint] Make flushPasspointAnqpCache API public

Make mWifiManager#flushPasspointAnqpCache() public, so that PO/DO
apps could call it.
CTS test already created in ag/13742071

Bug: 179731217
Test: atest WifiManagerTest WifiServiceImplTest
Change-Id: Ie48f6023cd379a04fdb8720d1b143783e3ba57c1
3 files changed