Add native font enumeration API implementation

ASystemFont API provides an ability of enumerating the system available
font files with some additional information.

ASystemFont reads /system/etc/fonts.xml and give the information to
developers.

Lots of developers reads fonts.xml by themselves but that is not a part
of public API and the structure is subject to change. This API is a
replacement of the reading fonts.xml by developers.

Bug: 111861508
Test: atest NativeSystemFontTest

Change-Id: I992902228650303eb10f486f1ec7838f218c15e7