Adding blank Android.mk to tools/bdk

Prevents tools/bdk from being built, including any
subdirs (in particular, downloaded but unlinked BSPs)

BUG: 28166939
TEST: Unit tests pass.
TEST: Ran builds from both in-tree and BDK.
Change-Id: I9fc029d350446d605886b8e833104955929ed9f0
2 files changed