Ensure that stubs.symbol_file has ".map.txt" suffix

The property refers to the symbol file that describes APIs across
updatable components like APEXes and system/vendor. Since it's important
to keep the APIs backwards incompatible, any change to the APIs should
be reviewed/gated by the NDK API council. The map.txt suffix is the key
for trigging the NDK API review by gerrit.

Bug: N/A
Test: m nothing
Change-Id: I01eae9c6a106d301863ec86e169111712af74d3f