updater: Add "write_value()" function.

write_value(value, filename) writes 'value' to 'filename'. It can be
used to tune device settings when applying an OTA package. For example,
write_value("960000", "/sys/devices/system/cpu/cpu0/cpufreq/scaling_max_freq").

Bug: 32463933
Test: recovery_component_test passes.
Test: Apply an OTA package that contains a call to write_value(), and
      check the result.

Change-Id: Ib009ecb8a45a94353f10c59e2383fe1f49796e35