Support LOCAL_RENDERSCRIPT_TARGET_API := current

Renderscript was using LOCAL_RENDERSCRIPT_TARGET_API := 0 to mean
current.  Use current instead, and check for it when comparing
if LOCAL_RENDERSCRIPT_TARGET_API >= 21.

Fixes an issue where LOCAL_RENDERSCRIPT_TARGET_API := 0 was being
treated as < 21, causing the renderscript resources to be passed
to aapt2:
out/target/common/obj/APPS/RSTestBackward_intermediates/flat-res/zip_res.flata.contents/raw/bc32: error: resource file cannot be a directory.

Bug: 79481102
Test: m java FORCE_AAPT2=true
Change-Id: I7726d0463ce1693bc9653b4d869ced181c491036