Rename RichContentReceiver.SOURCE_MENU to SOURCE_CLIPBOARD

Using "menu" was ambiguous since ACTION_PROCESS_TEXT is also triggered
via the menu but has its own enum value.

Bug: 152068298
Bug: 154151141
Test: ran tests
  atest CtsWidgetTestCases:TextViewRichContentReceiverTest
Change-Id: I3805577e9d7bbf55d40339d37820cd7f510da37e