Added number of receivers to ReceiverListProto.

Also added checks for null app on ReceiverList.

Bug: 226224551
Test: adb shell dumpsys activity --proto broadcasts receivers --uid 1000 > ~/tmp/proto.pb && \
  cd $ANDROID_BUILD_TOP && \
  printproto --proto frameworks/base/core/proto/android/server/activitymanagerservice.proto --message com.android.server.am.ActivityManagerServiceDumpBroadcastsProto --raw_protocol_buffer ~/tmp/proto.pb

Change-Id: I2ef27878a416fb388a7516ad88803f4965c36525