[MTE] Enable ASYNC MTE in the Zygote.

This allows applications to turn on MTE. The current design of MTE
inside of Scudo doesn't allow us to late-enable MTE (but we can
late-disable it). Thus, we have to enable tagging in the Zygote in order
to allow apps and the system_server to control their own MTE behaviour.

We intentionally use ASYNC mode so that we don't waste memory collecting
stack traces from the zygote pre-fork. Given that the zygote is mostly
Java code, the process itself should be pretty low risk for memory
corruption.

Bug: 135772972
Bug: 172365548
Test: On FVP: atest -s localhost:5555 \
com.android.cts.tagging.TaggingSdk30Test#testMemoryTagChecksCompatFeatureEnabled \
com.android.cts.tagging.TaggingSdk30Test#testMemoryTagChecksCompatFeatureDisabled

Change-Id: I9da2c33cb6022033264f443308d5508b3ff8da9e