use @databasename  is your answer, so in your case it will be something like( you will need to pass database name):
USE ' + @databasename + '
Insert into master.dbo.[Index_Frag] (
Database_Name
,Eschema
,TableName
,IndexName
,[Avg_Fragm]
,page_count
)

SELECT '''+@databasename+''',
.....................
FROM sys.dm_db_index_physical_stats (db_id(''' + @databasename + '''), NULL, NULL, NULL, NULL)
........................