Use different StoryBook font-size and letter-spacing per ReadingLevel

[nya-elimu]

StoryBooks are categorized into 4 ReadingLevels. Children on level 1 (just beginning to read) will be presented with larger font size, larger spacing between words, and larger spacing between letters. Then, as they gradually improve their reading skills, we can reduce the font-size and letter/word spacing.

Add adjustments for reading level 2, 3 and 4.

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Relates to https://github.com/elimu-ai/vitabu/issues/35 Relates to #1166

[nya-elimu]

@Keerthi4308 For reference, see the implementation of this issue in the Android app: https://github.com/elimu-ai/vitabu/issues/35

In particular, dimens.xml: https://github.com/elimu-ai/vitabu/commit/be739a67ae29999165def8ee9b7020618b8dff69#diff-98f128474b38c601bb720f8dced679d72d2ed0b2dbd1212cece74daa810e269c