apex: Install nativeTest with soong_cc_prebuilt.mk

This commit switches the build rule for APEX nativeTests from
`$(BUILD_PREBUILT)` to `soong_cc_prebuilt.mk`.  Since these are not real
prebuilts, `soong_cc_prebuilt.mk` should be sufficient.

Using `soong_cc_prebuilt.mk` also fixes a problem reported by prebuilt
ELF checker because `LOCAL_SHARED_LIBRARIES` are not generated for
nativeTests.

Bug: 139525855
Test: lunch aosp_walleye-userdebug && make check-elf-files
Change-Id: I7bfc29f9c2708896dad4e7cfc214480e1205a51e