Build custom empty SkFontMgr for Android Framework

Bug: b/118742766

The Android Framework does not need SkFontMgr_android*. Switch it to
using SkFontMgr_custom*. This will also be the SkFontMgr to use when
we're building Skia as a host-side library.

BUILD.gn:
- Rename fontmgr_fuchsia to fontmgr_custom_empty so it can be selected
  specifically by the Android Framework build. Default condition is
  unchanged, so fuchsia will still build it.
- Similarly, create a new argument for fontmgr_android and
  fontmgr_custom, so they can be omitted by the Android Framework build.
  Again, default condition is unchanged, so no other builds should be
  affected.

gn_to_bp.py:
- Set the new arguments to build the files we want.

Change-Id: Ic85db8acd64f625b00fe2c9bc451ae8814d97a59
Reviewed-on: https://skia-review.googlesource.com/c/170725
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Leon Scroggins <scroggo@google.com>
2 files changed