hotspot2: expose string constants for cert type and auth methods

So that we can use these constants in WifiService and the apps
as well in the future.

While there, remove auth method "CHAP" from the supported list,
since it is currenlty not supported by WifiService.

Bug: 34206769
Test: make -j32
Change-Id: Icadc3b13f2b135d51445d336116fbba86e96b70f