Rewrite how to generate apexkeys.txt

Instead of listing all apexes in the source tree, now each apex emits
its own fragment for apexkeys.txt, which is pointed by
LOCAL_APEX_KEYS_FILE. Makefile collects apexkeys.txt from installed apex
files. This is to avoid listing unrelated apexes (not installed,
testdata, unexported namespaces, etc.)

Bug: 304914238
Test: m apexkeys.txt
Test: m blueprint-tests
Change-Id: I6b5601609d16452a0717f09ecaa703ee09693094
diff --git a/core/clear_vars.mk b/core/clear_vars.mk
index 409e559..d76c4f2 100644
--- a/core/clear_vars.mk
+++ b/core/clear_vars.mk
@@ -19,6 +19,7 @@
 LOCAL_ALLOW_UNDEFINED_SYMBOLS:=
 LOCAL_ANNOTATION_PROCESSORS:=
 LOCAL_ANNOTATION_PROCESSOR_CLASSES:=
+LOCAL_APEX_KEY_PATH:=
 LOCAL_APIDIFF_NEWAPI:=
 LOCAL_APIDIFF_OLDAPI:=
 LOCAL_APK_LIBRARIES:=