Update expectations around APEX manifests in `art_apex_test.py`.

Recent changes broke ART's Host APEX checks. Adjust and increase them by:
- checking for the Protocol Buffers APEX manifest (in all APEXes);
- no longer checking for the (Android Q compatible) JSON APEX
  manifest (even in target APEXes).

Test: art/build/apex/runtests.sh
Bug: 143654022
Bug: 139277987
Change-Id: I4afc23d56fe90aaf447f334bde3bc0117f12d1aa
diff --git a/build/apex/art_apex_test.py b/build/apex/art_apex_test.py
index 7d2c79a..630a571 100755
--- a/build/apex/art_apex_test.py
+++ b/build/apex/art_apex_test.py
@@ -470,8 +470,8 @@
     return 'Release Checker'
 
   def run(self):
-    # Check the APEX manifest.
-    self._checker.check_file('apex_manifest.json')
+    # Check the Protocol Buffers APEX manifest.
+    self._checker.check_file('apex_manifest.pb')
 
     # Check binaries for ART.
     self._checker.check_executable('dex2oat')
@@ -562,6 +562,11 @@
     return 'Release (Target) Checker'
 
   def run(self):
+    # We don't check for the presence of the JSON APEX manifest (file
+    # `apex_manifest.json`, only present in target APEXes), as it is only
+    # included for compatibility reasons with Android Q and will likely be
+    # removed in Android R.
+
     # Check binaries for ART.
     self._checker.check_executable('oatdump')