Make wifi hidl interfaces available to the platform

The wifi hidl interfaces need apex_available: ["com.android.wifi"]
because the generated java interfaces are needed by the
com.android.wifi apex, but that causes the C interfaces to not
be available to the platform.  The unavailability was hidden
by b/154888298.  Explicitly mark them with "//apex_available:platform".

Bug: 154888298
Test: m checkbuild
Change-Id: I8959ec1454cce6817802bcc1785a34ba56adc99c