Bump third_party/mbedtls/repo from `5bc604f` to `9aa9120` #2210

Job	Run time
Build on Linux (gcc_debug)	1h 3m 48s
Build on Darwin (clang, python_lib, simulated)	1h 33m 29s
Build on Linux (fake, gcc_release, clang, simulated)	2h 6m 0s
Build on Linux (coverage)	16m 14s
Build on Linux (python lighting-app)	7m 18s
Build on Linux (python_lib)	24m 38s
	5h 31m 27s

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