Move file_contexts file for APEXes to under /system/sepolicy

For centralized development of sepolices, file_contexts files for APEXes
are all moved to under /system/seplicy.

The meaning of the existing file_contexts has been changed; when it is
set to <value>, then /system/sepolicy/apex/<value>_file_contexts is
used. When unset, it defaults to the name of the module. The property is
not removed in order to support creating multiple versions of the same
apex (for testing purpose) built with same file_contexts file.

Bug: 119034475
Test: m apex.test com.android.tzdata com.android.runtime
Change-Id: I7d14a9e37baea9ab78d9e15e1164cce54d256f56