Allow apps to process ProxyFDCallback asynchrnously.

Previously callback methods of ProxyFileDescriptorCallback were invoked
on a background thread prepared in the framework. So all methods were
invoked and processed synchronously. This was problem because if it took
time to fetch bytes of one file, operations for other files were also
blocked.

The CL changes ProxyFileDescriptorCallback methods to be invoked on
Handler passed by apps. Now application can prepare a Handler per file
so that one file does not block others.

Bug: 35229514
Test: cts-tradefed run cts -m CtsOsTestCases -t android.os.storage.cts.StorageManagerTest#testOpenProxyFileDescriptor_async
Change-Id: Ibadc4aad4c0373a3da586459a8f775e40288c895