[MTE] Parse MTE sysprop in system_server

The system server inherits the ELF note from the Zygote, but should also
be able to be overwritten by the system property. This needs to be
manually parsed.

Bug: 172365548
Test: 'adb shell setprop arm64.memtag.process.system_server 123' \
Test: ... kill the system_server, and and look in logcat for
Test  ... "Unknown memory tag level for the system server".
Test: 'adb shell setprop arm64.memtag.process.system_server sync' \
Test: ... kill the system_server, and look in logcat for
Test: ... "SetHeapTaggingLevel: tag level set to 3".

Change-Id: Icda4ff141646086ae85d751b8af9b1fe94bfd7b5