Mark abs/labs/llabs as 19+ rather than 21+

The libc.map.txt file already defines these three functions as having
been introduced in API 19, but these header files had declared them as
21 and up.

See https://android-review.googlesource.com/c/platform/development/+/233407

imaxabs is already declared as 19 and up.

Test: build-ndk-prebuilts.sh && ./update_platform.py &&
  checkbuild.py && run_tests.py
Test: manually build app against 18 and 19

Change-Id: Iaeed48d7e6c438d816635a0433a056e557e8ebc2