Introduce --report-timings flag for flutter build aot command.

[mraleph]

Introduce --report-timings flag for flutter build aot command.

This flag makes flutter build aot report timings for substeps (e.g.
frontend compilation and gen_snapshot) in a machine readable form.

Previously our tools for continuous performance tracking attempted to
parse verbose output of flutter build aot -v and derive timings
from it - but this is extremely fragile.

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[mraleph]

/fyi @aam

[jonahwilliams]

LGTM