Update BUILD_ID

Master branch has been renamed to main, the build id needs to reflect
this change.

Ignore-AOSP-First: Merge conflict resolutions
Bug: 290400015
Change-Id: I740ef3225c4c291ef7313d3ff1fe5d78389497f5
(cherry picked from commit f0898eae0755cf5c66a72dda14aab28760e51639)
Merged-In: I740ef3225c4c291ef7313d3ff1fe5d78389497f5
diff --git a/core/build_id.mk b/core/build_id.mk
index a489788..bed839f 100644
--- a/core/build_id.mk
+++ b/core/build_id.mk
@@ -18,4 +18,4 @@
 # (like "CRB01").  It must be a single word, and is
 # capitalized by convention.
 
-BUILD_ID=TM
+BUILD_ID=MAIN