Add a new Soong config variable EMMA_INSTRUMENT

This change introduces a new Soong config variable named
`EMMA_INSTRUMENT`. This variable mirrors the behavior of the existing
`EMMA_INSTRUMENT` condition in Android.mk files, allowing for
conditional build configurations based on whether EMMA instrumentation
is enabled.

Bug: 342330305
Test: 1) EMMA_INSTRUMENT=true m ahat-tests
      2) m -j ahat-tests
Change-Id: I7bc28cd3de936d1e6bfb218683bcf07b40e80784