Introduce set/getFontVariationSettings.

This CL enables developers to specify axis values to the underlying
font collection. The specification of the font variation settings is
the same as the CSS font-variation-settings attribute in CSS working
draft as of 2016-11-30.

Code example: Here is an example to set width 100 and weight 1.5.

  TextView tv = (TextView) findViewById(R.id.textView);
  tv.setFontVariationSettings("'wdth' 100, 'wght' 1.5");

Bug: 33062398
Test: Manually done. Ran FrameworksGraphicsTests, CtsGraphicsTestCases
      and CtsWidgetTestCases
Change-Id: I249d464f8cdaa56017a987588b94ed685aadeb58