Symbol files describing APIs across updatable components are reviewed

The build system will soon require that symbol files describing APIs
across updatable components (e.g. across APEXes or across sytem/vendor)
have the ".map.txt" suffix. This will trigger the NDK API review in the
gerrit so that backwards incompatible changes are prevented.

Bug: N/A
Test: m nothing
Change-Id: I114563782f8e796365ca141eb5d22d2e203a7f55