Shrink Mac builds

Device builds are no longer supported on Mac, but we do support building
various host tools, including the SDK build-tools and platform-tools
packages. All of these are defined in Soong now, so we stop reading all
the Android.mk files, and defining and depending on the device output
targets.

Bug: 187222815
Test: `m`, `m dist`, etc on Mac
Change-Id: Ibb19756d44f2cbc6948381043115718ffc3f4b7d