framework-wifi: default build rule for tests

Create a java_default build rule for tests that
need to build against @hide APIs from
framework-wifi.

Visibility is restricted to select tests for now.

Also restricted visibility for framework-wifi
itself.

Bug: 146513037
Test: compiles
Change-Id: Ia6dec8cf0202e0417d54a480c1715190b03e5e3e