Introduce tabs to view message, attachment and invite separately

Added "tabs" to the message view according to the latset mock.
This removes the necessity of putting a WebView inside a ScrollView,
which caused the infinitely-growing email bug (issue 6882).

Right now the tabs are actually just Buttons.  Complete visual refresh
should follow it.
Bug 2349275

Change-Id: I897a3a32e0dd7a90d637ac5ea1d47e5e65a1eabe
1 file changed