Load font file from remote provider synchronously.

Since CancellationSignal is not guaranteed to be handled by remote
process, introduce threaded font loading mechanism.
At the same time, this fixes possible NPE issue in openFont.

Bug: 37552479
Test: am instrument -w -e class android.provider.FontsContractE2ETest
      com.android.frameworks.coretests/\
      android.support.test.runner.AndroidJUnitRunner

Change-Id: Ie235c5a57bd172b0882da5aa27a990d7c6686ff2