[AWARE] Move WifiAwareNetworkSpecifier Builder to its parent class

Note: API only change - no functional changes

Move the Builder class and method which generates WifiAwareNetworkSpecifier
to the class it actually builds and have it return that type.

Per API council feedback to match standard builder patterns.

Bug: 126700127
Test: builds + make docs
Test: atest com.android.server.wifi.aware
Test: atest android.net.wifi.aware
Test: atest SingleDeviceTest (CTS)
Change-Id: If4d58e96b4a268df7afe58bc3d883b6c54ec889d