Reduce unnecessary overhead of screenshot bitmap from assist

Since hardware bitmap is introduced, it is already fast enough to
pass across processes. And Bitmap#asShared becomes an unnecessary
operation.

This can save 70%+ execution time of requestAssistScreenshot.
About from 65ms to 15ms.

Bug: 201351584
Test: Measure the time from a test app with looping
      IWindowManager#requestAssistScreenshot(receiver).

Change-Id: Ie5510eae789219e3afbe1a9ecf8911925207c0ba