Implement family fallback.

Introduce new attribute "fallbackFor" to font element.
By specifying name of the family to this attribute, that font is used
when the developer specifies the font family.
For example, if fonts.xml has the following family entry,
  <family lang="ja">
    <font fallbackFor="serif">NotoSerifJP-Regular.ttf</font>
    <font>NotoSansJP-Regular.ttf</font>
  </family>
the Japanese text is rendered by NotoSansJP-Regular.ttf by default.
Then, if developer specifies fontFamily="serif" in TextView, the Japanese
text is rendered by NotoSerifJP-Regular.ttf.

Bug: 37328609
Bug: 31491668
Test: bit FrameworksCoreTests:android.graphics.TypefaceSystemFallbackTest

Change-Id: I2744db7384c8056795e841c88b387545434131f4