Propagate LTO type from binary/DSO to object files

Propagation of LTO flags from a main binary or library to its static
dependencies is necessary so that objects being linked with LTO are built as
bitcode rather than native objects. This propagation was broken when thin LTO
was added to the build system. This patch propagates the LTO mode (thin or full)
down to object and archive dependencies.

Test: Build libhwui with full and thin LTO and check that intermediate objects
are LLVM bitcode.
Change-Id: Ic57a80d75052d86940db076fbc0f5375a79c89a1