Update APN proxy beforer network agent created.

When APN proxy setting exists, proxy is not properly
applied to network agent often.

When state is chagned to ActiveState in DataConnection,
1) DcTracker sets http proxy into LinkProperty (by event),
2) and NetworkAgent is created with current LinkProperty.
But the thing is the sequence of 1), 2) is not deterministic.

Therefore,
- Move proxy setting logic from DcTracker to DataConnection
- Update proxy setting just before NetworkAgent is created

Test: manually tested on emulator behind proxy server
Test: step1. create new APN with proxy
Test: step2. turn on/off flight mode
Test: step3. connect to google.com using webview
Bug: 156527346

Change-Id: I7ecaa9eaaca419b75b7645ae7523dafa4de8c0be
Signed-off-by: Wonguk Jeong <wonguk.jeong@samsung.com>