Move font tool build rules to subdir

The top-level Android.bp file is very large, so move modules that can be
in subdirs.

Also rename the vague "base_defaults" module to something more suitable.

Bug: 185128417
Test: m
Merged-In: I5f3106b74edb91ac9db5b3e66cc61566d0f16e9b
Change-Id: I5f3106b74edb91ac9db5b3e66cc61566d0f16e9b