If the library does not provide system and test APIs then do not
generate/require corresponding .txt files.
Bug: 145998881
Test: m checkbuild
Change-Id: I21cfdb0b63fd575e8c8c63ea2b436e0c4aa8f3fc
Some non-interpretable error messages has been shown in case of missing
current api files. Instead of that messy errors, This clearly tells
users what is the problem and how to fix it.
Bug: 126259114
Test: 1) try to build a sysprop_library and a java_sdk_library without
creating txt files under api/, and see it fails.
Test: 2) Copy and paste the emitted command, and try to build again.
Change-Id: I986781cc710475647c2b996b0f1c51a6f50c0ba2
