Implementing new API : takeScreenshot()

Implementing the new API for A11y services to take the screenshot
of the specified display.
Bitmap takeScreenshot(int displayId)
1. The main codes is moved from the UiAutomationConnection class and
let the UiAutomation change to use this API.
2. Add a capability at metadata to check whether the A11y services could use this
API or not.
3. This API is a async one for A11y services, but is a sync one for
UiAutomation.

Bug: 10931661
Test: a11y CTS & unit tests
Change-Id: I478bd93c60d4742bef20ae0e423ca9de1bda55d2