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/base_rules.mk b/core/base_rules.mk
index 3313b5f..0eb205a 100644
--- a/core/base_rules.mk
+++ b/core/base_rules.mk
@@ -1180,6 +1180,7 @@
 ALL_MODULES.$(my_register_name).TEST_MAINLINE_MODULES := $(LOCAL_TEST_MAINLINE_MODULES)
 ifndef LOCAL_IS_HOST_MODULE
 ALL_MODULES.$(my_register_name).FILE_CONTEXTS := $(LOCAL_FILE_CONTEXTS)
+ALL_MODULES.$(my_register_name).APEX_KEYS_FILE := $(LOCAL_APEX_KEY_PATH)
 endif
 ifdef LOCAL_IS_UNIT_TEST
 ALL_MODULES.$(my_register_name).IS_UNIT_TEST := $(LOCAL_IS_UNIT_TEST)