Add LOCAL_LOGTAGS_FILES that prebuilts can use

Soong modules can define logtags files that need to be combined into
/system/etc/event-log-tags, so add a new LOCAL_LOGTAGS_FILES variable
that can be used to specify *.logtags file outside of LOCAL_SRC_FILES.

Bug: 28989759
Change-Id: I53c5d396dfb7c6006806758f351eb5cdde90fe74
diff --git a/core/clear_vars.mk b/core/clear_vars.mk
index bc1eefb..9ca880b 100644
--- a/core/clear_vars.mk
+++ b/core/clear_vars.mk
@@ -197,6 +197,7 @@
 # to avoid installing the presigned apks with classes.dex unstripped.
 LOCAL_REPLACE_PREBUILT_APK_INSTALLED:=
 LOCAL_NDK_VERSION:=current
+LOCAL_LOGTAGS_FILES:=
 
 # arch specific variables
 LOCAL_SRC_FILES_$(TARGET_ARCH):=