Executables in APEXes have root:shell

Executables under /system/bin are configured as (uid:gid)=(root:shell)
by fs_config.cpp. Therefore, an executable that is moved/copied from
/system/bin to an APEX should be configured as such.

Test: adb shell ls -al /system/bin/linker
/apex/com.android.runtime/bin/linker shows
-rwxr-xr-x 1 root shell 1133528 1970-01-01 09:00 /apex/com.android.runtime/bin/linker
-rwxr-xr-x 1 root shell 1133528 1970-01-01 09:00 /system/bin/linker

Change-Id: Ibb698aab237362fed312da2af809cceead8a1092