[Partial Screensharing] Rename start button id

Rename start button id to android:id/button1 so that CTS test could work for both with and without PSS (where AlertDialog is used)

HSV: https://screenshot.googleplex.com/7JDQNXDTFPp6Voo.png

Fixes: 264853338
Test: atest CtsMediaProjectionTestCases:MediaProjectionTest
Change-Id: I642f91d9d91c10fb4248a5eb02f92d9c523e00e9
2 files changed