Set sys.platform value for hermetic Python.

sys.platform is determined at compile/build time. By default, CPython
will give it a "unknown" value if we didn't set the cflag.

Test: adb push device_test /data, adb shell, ./device_test
Bug: b/70218680
Change-Id: If5d914d3c43b1cfd3df1e57c81becacf2ac59e36
1 file changed