Fix cache config bug

[sidrao2006]

Breaking Changes and Migration Guide

1. If you have been using the package without modifying cacheStalePeriod or maxCacheObjects, no changes are required.
   However, any previously cached font files will be ignored and should be deleted by running the migration tool.

2. If you have modified cacheStalePeriod or maxCacheObjects, you'll have to pass the same values to any method that downloads, caches or loads fonts, otherwise the provided configuration will be ignored.
   Any previously cached font files will continue to remain in their respective cache folders and will be used by the package.
   Running the migration tool will have no effect on these font files.

3. A migration tool has been provided -> DynamicCachedFonts.runMigrationTool()

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Original behaviour: A default cache manager with cache key 'DynamicCachedFontsFontCacheKey' was used which stored font files in a folder with the same name.
If a value for cacheStalePeriod or maxCacheObjects is provided which differs from the default value then a new instance of CacheManager would be created with the generated cacheKey.

Problems:

1. Once a garbage collection event is run, _cacheManagers is destroyed and hence the package forgets about the modified CacheManager and attempts to locate the file in the default cache folder (as mentioned above).

2. CacheManager requires a singleton approach. Since each cacheKey requires a unique instance of CacheManager, calling any method provided by the package before calling cacheFont/cacheFontStream would result in cacheStalePeriod, maxCacheObjects to be ignored when downloading and adding the font to cache.

Solutions:

1. There will be no default instance of CacheManager, instead an instance will be created using cacheKey after any garbage collection events (which includes app restarts).

2. Any method that downloads, caches or loads font will now accept cacheStalePeriod, maxCacheObjects . This ensures that an instance of CacheManager will be created only once with the provided config

Changes:

• Require cacheStalePeriod, maxCacheObjects to be passed to any
  method that calls getCacheManager. As a result, if the above mentioned
  properties are modified by the user of the package at any point, the
  modified values should be passed to any method provided by this package
• Require cacheStalePeriod, maxCacheObjects to be passed to getCacheManager
• Remove handleCacheManager

See #233