toolchain: Switch back to tracked revisions on Android

This allows us to recreate the entire LVL package from any point in history.
We continue to pull known good revisions from AOSP.
diff --git a/build-android/spirv-headers_revision_android b/build-android/spirv-headers_revision_android
index 1f7391f..d3fb69e 100644
--- a/build-android/spirv-headers_revision_android
+++ b/build-android/spirv-headers_revision_android
@@ -1 +1 @@
-master
+e9f7d8ac3cda59f2637776eea1dfad0e171fba21