A few layout improvements + more

* AndroidManifest.xml, styles.xml
 - New theme for the "Add Homework" Activity for tablets, now shows as a floating dialog, as there really wasn't enough content for a full screen Activity. 
 - Uses `android:theme="@style/AddTheme"`
 - Small devices - redirects to `"AppBaseTheme"`
 - Tablets - Uses this theme: `"android:Theme.Holo.Light.DialogWhenLarge"`

* activity_add.xml
 - Removed unnecessary TableLayout, what you changed can be done in a LinearLayout.
 - Moved the "Add" button to a floating Button Bar, compare to the Package Installer, using this:
`<include layout="@layout/button_bar"/>`

 - I found that even though your minSdkVersion was 4, Android 1.6, you were using `"match_parent"` which was added in API 8, or Android 2.1. Instead, I used `"fill_parent"`, deprecated, but backwards-compatible. Eclipse may yell at you. Ignore it. 

* activity_list.xml
 - Changed `"match_parent"` to `"fill_parent"`.

* button_bar.xml - Button bar at the bottom that looks like dialog buttons - Two different layouts:
 - layout: Uses Gingerbread style for Gingerbread and lower, using `android:background="@android:drawable/bottom_bar"` and a standard Button.
  - layout-v11: Uses the new `style="android:attr/buttonBarStyle"` and `style="android:attr/buttonBarButtonStyle"` for Honeycomb and above.

* Preferences.java
 - The path for the preferences file was still hardcoded. I fixed it.