Only spawn one binder thread for screencap.

To work around race conditions in libbinder when
spawning new threads while being shut down.

Bug: 36066697
Test: screencap still works
Change-Id: I774d3dc2491bb578a12dede90cc7fcded976f13f

cmds/screencap/screencap.cpp

@@ -84,6 +84,11 @@ static status_t notifyMediaScanner(const char* fileName) {
 
 int main(int argc, char** argv)
 {
+    // setThreadPoolMaxThreadCount(0) actually tells the kernel it's
+    // not allowed to spawn any additional threads, but we still spawn
+    // a binder thread from userspace when we call startThreadPool().
+    // See b/36066697 for rationale
+    ProcessState::self()->setThreadPoolMaxThreadCount(0);
     ProcessState::self()->startThreadPool();
 
     const char* pname = argv[0];