Do not use many FDs in FontManagerService.

Currently there are over 170 font files are installed in system directory.
Opening 170+ files and keep them is not unacceptable. Pass URI instead.

At the same time, this CL hides full font path from FontConfig since
/system/fonts directory will be deprecated in future.

Bug: 36660849
Test: android.text.cts.FontManagerTest passed
Change-Id: I1d216dc9c6dec702a4ce3b946bfda6dcbe12b7fe