Split property and file contexts modules

1. Splitted plat_property_contexts and plat_file_contexts so that they
can be included by the CtsSecurityHostTestCases module;
2. Add temporary seapp_contexts Soong module, which are needed by the
CtsSecurityHostTestCases, and makefile_goal is an interim solution before
migrating both of them to Soong.

Bug: 194096505
Test: m CtsSecurityHostTestCases
Change-Id: I0e0e7f6778d42ab2fdae3a181f40216fe6719e7c
2 files changed