Update FontsContract.requestFonts

- Change FontsContract.requestFont plural form to meet other method
naming convension.
- Change FontsContract.requestFonts to accept CancellationSignal to
be able to notify to remote.

Bug: 37326126
Test: am instrument -w -e class android.provider.FontsContractE2ETest
      com.android.frameworks.coretests/\
      android.support.test.runner.AndroidJUnitRunner

Change-Id: I154f943eb4e7c91bff8afd86be479f72e66e5cb7