android: Handle GC cycle handlers not being exported (#330)

Which happens on some custom ROMs. This does however mean that our hooks
become unreliable on such systems, since we fail to tie into the GC
cycle. We should consider scanning libart's memory as a fallback.

lib/android.js

@@ -1894,21 +1894,27 @@ function ensureArtKnowsHowToHandleReplacementMethods (vm) {
   const apiLevel = getAndroidApiLevel();
 
   const mayUseCollector = (apiLevel > 28)
-    ? new NativeFunction(Module.getExportByName('libart.so', '_ZNK3art2gc4Heap15MayUseCollectorENS0_13CollectorTypeE'), 'bool', ['pointer', 'int'])
+    ? (type) => {
+        const impl = Module.findExportByName('libart.so', '_ZNK3art2gc4Heap15MayUseCollectorENS0_13CollectorTypeE');
+        if (impl === null) {
+          return false;
+        }
+        return new NativeFunction(impl, 'bool', ['pointer', 'int'])(getApi().artHeap, type);
+      }
     : () => false;
   const kCollectorTypeCMC = 3;
 
-  if (mayUseCollector(getApi().artHeap, kCollectorTypeCMC)) {
+  if (mayUseCollector(kCollectorTypeCMC)) {
     Interceptor.attach(Module.getExportByName('libart.so', '_ZN3art6Thread15RunFlipFunctionEPS0_b'), artController.hooks.Gc.runFlip);
   } else {
-    let exportName = null;
+    let copyingPhase = null;
     if (apiLevel > 28) {
-      exportName = '_ZN3art2gc9collector17ConcurrentCopying12CopyingPhaseEv';
+      copyingPhase = Module.findExportByName('libart.so', '_ZN3art2gc9collector17ConcurrentCopying12CopyingPhaseEv');
     } else if (apiLevel > 22) {
-      exportName = '_ZN3art2gc9collector17ConcurrentCopying12MarkingPhaseEv';
+      copyingPhase = Module.findExportByName('libart.so', '_ZN3art2gc9collector17ConcurrentCopying12MarkingPhaseEv');
     }
-    if (exportName !== null) {
-      Interceptor.attach(Module.getExportByName('libart.so', exportName), artController.hooks.Gc.copyingPhase);
+    if (copyingPhase !== null) {
+      Interceptor.attach(copyingPhase, artController.hooks.Gc.copyingPhase);
     }
   }
 }