Prevent dial() return null in a racing condition.

The racing condition happens between dial() returns and
the first GET_CURRENT_CALLS query gets handled.

If GET_CURRENT_CALLS gets handled before dial() finishs, the pendingMO can be set
to null in handlePollCalls() so that dial() will return null. This null connection causes
error in PhoneUtils.placeCall().

The Synchronized dial() and handlePollCalls() Methods will make sure the
dial() returns before the first GET_CURRENT_CALLS gets handled.

bug:6028290
Change-Id: I41b024760acb7dd13b342866180dffe3fdbe1c03