Call launchGetThemesWebView if ActivityNotFound.

Themes Showcase may be installed but disabled by the user.  If this
is the case an ActivityNotFoundException will be thrown.  We catch
this exception and show the themes web view instead.

Change-Id: I5771215b27493bbfa8fa2643cd1e7c3ec2986ded
Merge branch 'cm-11.0' of https://github.com/CyanogenMod/android_packages_apps_ThemeChooser into HEAD