The proto build rules are quite large and makes the top-level Android.bp
file difficult to navigate. Move them to a separate file.

Bug: 185128417
Test: m
Merged-In: I5719ba0fa8e03dcfca0cc32dc30db3f87ea4e3cd
Change-Id: I5719ba0fa8e03dcfca0cc32dc30db3f87ea4e3cd