Change PreferenceCategory margins.

Top and bottom margins of the frame layout are changed to 8dp per bug
request. A 16dp top margin was added to the title in order to match
mocks provided in bug.

This also fixes the WifiPicker currently connectected access point to
make it centered.

Bug: 34978500, 34714183
Test: Visual. See screenshot in comments.
Change-Id: Idfa978b2defeb9b3f16651d670c5578f33b02656