[Tagged Pointers] Allow probing the current TP level w/ locking.

aosp/1484976 introduced a breaking change where
DisableMemoryMitigations() now indiscriminately turns tagged pointers
off. When android_mallopt(M_DISABLE_MEMORY_MITIGATIONS) is called, the
correct behaviour is:
 - In SYNC/ASYNC MTE mode -> disable all tagged pointers.
 - If all tagged pointers are already disabled -> nop.
 - If we're in TBI mode -> nop (keep the TBI mode as-is).

In order to do that we have to allow probing of the current heap tagging
mode. In order to prevent TOCTOU between GetHeapTaggingLevel() and
SetHeapTaggingLevel(), we expose a global mutex that should be held when
calling these functions.

Bug: 174263432
Test: atest CtsTaggingHostTestCases on Flame
Change-Id: Ia96f7269d542c9041270458806aee36766d2fbbb
diff --git a/TEST_MAPPING b/TEST_MAPPING
index ae61304..11efdae 100644
--- a/TEST_MAPPING
+++ b/TEST_MAPPING
@@ -17,6 +17,9 @@
     },
     {
       "name": "memunreachable_unit_test"
+    },
+    {
+      "name": "CtsTaggingHostTestCases"
     }
   ]
 }