aidl: add AssetFileDescriptor.aidl

- To allow this type to be used in structured AIDLs
- No SDK change. AIDL wasn't able to discover this type without an aidl
  file declaring it.

Bug: 408797213
Test: build
Flag: EXEMPT, no change
Change-Id: Ieecabd4b33e1191295465312b3dc2aa16081208a