Fix Android imports stamp file write mode

[cbracken]

'w' (write) and 'a' (append) are mutually exclusive open modes in python
-- 'w' truncates and opens for write; 'a' opens at end of file for
write; both create if the file doesn't exist.

Python 2.7 didn't warn about this; Python 3 does.

No new tests since the build is the test.

Issue: flutter/flutter#83043

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