Properly serialize access to FileSource::getSize by acquiring the mutex

Both ::getSize and ::read call into lseek64, if this happens simultaneously
from multiple threads the results are undefined if not properly serialized.

Change-Id: I737cafebd836f3d8eb702beac557b4731f69c6f6
related-to-bug: 5196490

media/libstagefright/FileSource.cpp

@@ -110,6 +110,8 @@ ssize_t FileSource::readAt(off64_t offset, void *data, size_t size) {
 }
 
 status_t FileSource::getSize(off64_t *size) {
+    Mutex::Autolock autoLock(mLock);
+
     if (mFd < 0) {
         return NO_INIT;
     }