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: Iefbe6e486cb418955584ad1a282455307e90be95

apex/androidmk.go

@@ -260,6 +260,7 @@ func (a *apexBundle) androidMkForType() android.AndroidMkData {
 				fmt.Fprintln(w, "LOCAL_SOONG_INSTALL_PAIRS :=", a.outputFile.String()+":"+a.installedFile.String())
 				fmt.Fprintln(w, "LOCAL_SOONG_INSTALL_SYMLINKS := ", strings.Join(a.compatSymlinks.Strings(), " "))
 			}
+			fmt.Fprintln(w, "LOCAL_APEX_KEY_PATH := ", a.apexKeysPath.String())
 
 			// Because apex writes .mk with Custom(), we need to write manually some common properties
 			// which are available via data.Entries