Fix NPE happens if no source was specified to font element.

This also fixes unexpected fallback to the old implemntation by calling
allowUnsupportedFont() which is only for backward compatibility.

This CL also remove getResourceId() method. Nobody uses this method.

Bug: 37865521
Bug: 37844248
Test: am instrument -w -e class android.content.res.cts.ResourcesTest\
      android.content.cts/android.support.test.runner.AndroidJUnitRunner
Change-Id: I987448add728c53c916f24a2ea8f337be417248f