Add mobi extensions to known mime types

Allow .prc/.mobi file extensions to be read as a separate
mobi mime type. This allows Android to open mobi files
with the correct mime type.

This in turn allows applications to set their intent filters
based on mime type when opening mobi files.

Test: CtsMimeMapTestCases
Change-Id: I2b8f80f019f930f888e8e50db8b4efee0f0d4035