Fix asset clean target.
Fix asset clean target.

"ndk-build clean" was running the "all" target in the asset build
process.  This change selects the "clean" mode of the asset build
script.  Unfortuntaely the asset build script's clean mode appears
to have rotted so all build assets are not cleaned correctly.

Change-Id: I4f8768001b39e26b8df7aa5e61934ff7a24be7b9

google/zooshi,google/zooshi,google/zooshi,google/zooshi,google/zooshi