Fix millisecond zero-padding in epoch_to_log_line_timestamp

[rootkiller6788]

Problem

mobly.logger.epoch_to_log_line_timestamp produced a non-zero-padded millisecond component:

return d.strftime('%m-%d %H:%M:%S.') + str(ms)

For epoch values whose millisecond remainder is below 100, str(ms) drops the leading zeros. For example epoch_to_log_line_timestamp(1469134262005, time_zone=datetime.timezone.utc) returned '07-21 20:51:02.5' instead of '07-21 20:51:02.005'.

This is inconsistent with the canonical log line timestamp format used everywhere else in the module: get_log_line_timestamp returns exactly three millisecond digits, logline_timestamp_re matches \d\d\d after the dot, and is_valid_logline_timestamp rejects anything else. Such malformed timestamps also compare incorrectly with logline_timestamp_comparator, which relies on fixed-width fields.

Fix

Zero-pad the millisecond component to three digits:

return d.strftime('%m-%d %H:%M:%S.') + f'{ms:03d}'

Test

Added test_epoch_to_log_line_timestamp_pads_milliseconds covering millisecond remainders of 0, 5, 50 and 99, asserting both the exact output and that is_valid_logline_timestamp accepts it.

python -m pytest tests/mobly/logger_test.py passes (27 passed).