First crash appeared to be caused by a failure to find the
`attr/colorBackgroundAccent` colour from the theme.

It wasn't clear why the attribute could not be found, so to fix it was
simpler to remove the color and attribute entirely, and replace it with
something more appropriate from the Material 3 tokens.

- Preview cards are stroked with `colorOutline`
- Poll options use `colorPrimary` (user's vote) or `colorSecondary`
  (other choices) with appropriate text colours.
- Links in link preview cards use `android:attr/textColorLink`
- The placeholder icon in preview cards uses `?android/textColorLink`
- Remove it from `help_message_background`, and stroke with
  `?colorOutline`

Doing this I discovered several places where a colour was being
specified unnecessarily, those have been removed.

To make it easier to understand the theme hierarchy that has been
collapsed and renamed to follow Android conventions.

- AppTheme -> Base.Theme.Pachli
- BaseTheme -> Theme.Pachli
- DefaultTheme has been removed as unnecessary

This unearthed a second crash, where `attr/actionBarSizeWithSubtitle`
was not found.

To fix that create an explicit style for toolbars that need it, and
apply the style (`Pachli.Widget.Toolbar`).

This also surfaced a third problem, where the `fragment_timeline*`
layouts had not been updated in `layout-sw640dp`, so those have been
updated to reflect the same views/IDs as the default `fragment_timeline`
layout.

These changes caused a small chain of "unused resource" lint errors,
which have been fixed by removing the unused colours.

The Android Material libraries were also being implicitly depended on
through other library imports instead of being explicit. So include
them as an explicit dependency.

Fixes pachli#18