Auto-clean host tools when switching between apps_only and platform build.

For apps_only (also PDK build) we use prebuilt host tools in
prebuilts/sdk/tools; For platform build we use tools built from source.
Auto-clean intermediate files of these tools when build type change is
detected.

Bug: 20213206
Change-Id: I9173af322684c017fdb91a3abfbe39ecfe5650e9