Convert BUILD_PREBUILT with LOCAL_MODULE_CLASS=ETC to prebuilt_etc

The conversion is a two-step process: first, when processing
BUILT_PREBUILT, convert LOCAL_SOURCE_PATH to a variable reference+fixed
subpath path in the blueprint AST. Then, set various boolean flags
depending on variable being referenced. androidmk_test.go has a test for
each handled case.

Change-Id: Iabd18d5f8645ca7077536863cd6640df5b24d24a
Fixes: 122906526
Test: treehugger