cargo2android: Support exported C headers

staticlib and cdylib crate types generally want to export header files
for use by C code. Allow the location of these headers to be provided to
c2a for inclusion in blueprint.

Bug: 178565008
Test: Generate rustc-demangle-capi
Change-Id: I0aa9acfb081ea39a7e13315bfc8029d06b8f55d3
1 file changed