Improve caching of Bitmaps by adding an in-memory LRU cache

[eduardb]

Right now, thumbnails that appear on the metadata screen are cached on disk, using the decodeSampledBitmapAndCache() method from the BitmapReader class. This could be improved further by adding an intermediate layer of caching, in-memory LRU cache, implemented using the androidx.collection.LruCache class from AndroidX.

This cache could also be used for the blank Bitmap that currently gets created every time a mask is saved, here:

GDD-app/app/src/main/java/net/aiscope/gdd_app/ui/mask/MaskActivity.kt

Line 199 in 20117b4

Bitmap.createBitmap(maskBitmap.width, maskBitmap.height, maskBitmap.config)

An additional requirement would be for this in-memory cache to react to low memory conditions. This can be achieved by overriding the onTrimMemory() method in the Application class, and depending on the level, evict an adequate number of bitmaps, using the trimToSize() method.

Relevant Android guide: https://developer.android.com/topic/performance/graphics/cache-bitmap#memory-cache