Automatically convert target API from 25 to 24.

  - There is no new APIs introduced for 25 and devices shipped with
  N-MR1 and N-MR2 can only support 24 or lower. This change will
  automatically convert target API 25 to 24.

Bug: 35767071
Test: mm and manually invoked llvm-rs-cc with -target-api 25 and
verified that it is converted to 24.

Change-Id: I1da7c94a0a594633c949d6ce080893d2b6027fc6
2 files changed