Support loading system_ext_property_contexts

Commit I27dd391fc06a3c78e88a65c7931c84de1699f157 makes property_service
no longer load selinux property_contexts by function
selinux_android_prop_context_handle.

So we should load property_contexts from /system_ext as well in
property_service.cpp.

Bug: 137712473
Bug: 141871131
Test: `adb shell getprop -Z` to check system_ext_property_contexts is loaded
Change-Id: I49c5c725784c0d598062ae356be6226446f4942a