Stop holding mProxyLock while downloading PAC script data.

Holding PacManager#mProxyLock for long time eventually causes ANR when
launching new applications.

1. ActivityThread#handleBindApplication blocks until

   ActivityThread#handleBindApplication
   -> ConnectivityService#getProxyForNetwork()
   -> ConnectivityService#getDefaultProxy()

  obtains the lock of ConnectivityService#mProxyLock

2. ConnectivityService#mProxyLock can be held by
   ConnectivityService#setGlobalProxy() running on another thread until

   ConnectivityService#setGlobalProxy()
   -> ConnectivityService#sendProxyBroadcast()
   -> PacManager#setCurrentProxyScriptUrl()

  obtains the lock of PacManager#mProxyLock

3. Before the CL, PacManager#mProxyLock could be held by
   mPacDownloader#run() on mNetThread until downloading a PAC script
   completed.

The CL fixes the step 3 so that mPacDownloader#run does not keep the
lock of PacManager#mProxyLock. It eventually fixes long blocking at
ActivityThread#handleBindApplication and ANR when launching an
application.

Bug: 36317236
Test: Build succeeded
Merged-In: I864e41e1142178681f8f1b4fb7750d37e3ab2076
Change-Id: I864e41e1142178681f8f1b4fb7750d37e3ab2076