Introduce system font enumeration API

To be able to use font file in their apps, provides blob and path to the
font file and locale list as well.

Bug: 26116537
Test: atest CtsWidgetTestCases:EditTextTest
    CtsWidgetTestCases:TextViewFadingEdgeTest
    FrameworksCoreTests:TextViewFallbackLineSpacingTest
    FrameworksCoreTests:TextViewTest FrameworksCoreTests:TypefaceTest
    CtsGraphicsTestCases:TypefaceTest CtsWidgetTestCases:TextViewTest
    CtsTextTestCases FrameworksCoreTests:android.text
    CtsWidgetTestCases:TextViewPrecomputedTextTest
    CtsGraphicsTestCases:android.graphics.font

Change-Id: I1ae1302c6906b808012e1e91b1e4ab393c887cb6