Fix incorrect use of "# introduced=" tag.

The tool that creates a stub from a map file (build/soong/cc/symbolfile)
requires that the introduced flag be in the form of "# introduced=31",
and doesn't recognize "#introduced = 31".

Bug: 177365934
Test: m
Change-Id: I38ccd39c06f05affd66b47f4e4182a3882ff7661