Android.mk

This fixes the following issues introduced in commit
d225b697:
* plat_file_contexts was empty because the target was referencing
  system/sepolicy/private/file_contexts via a misspelled variable
  name.
* plat_file_contexts wasn't marked as dirty and thus wasn't rebuilt
  when system/sepolicy/private/file_contexts changed. This is because
  the file_contexts dependency was referenced via a misspelled
  variable name.
* plat_file_contexts wasn't sorted (as opposed to other similar
  targets, such as nonplat_file_contexts and file_contexts.bin). This
  may lead to unnecessary non-determinism.
* nonplat_file_contexts wasn't marked dirty and thus wasn't rebuilt
  when device-specific file_contexts file(s) changed. This is because
  the file_contexts files were referenced via a misspelled variable
  name.

Test: "make plat_file_contexts" produces a non-empty file containing
      mappings from system/sepolicy/private/file_contexts
Test: "make plat_file_contexts" updates output when
      system/sepolicy/private/file_contexts changes
Test: "make plat_file_contexts" produces output which is sorted
      accroding to rules in fc_sort
Test: "make nonplat_file_contexts" updates output when
      device/lge/bullhead/sepolicy/file_contexts changes (tested on
      aosp_bullhead-eng)
Bug: 31363362
Change-Id: I540555651103f02c96cf958bb93618f600e47a75