Add ability to configure max cache size and and count for ParseKeyValueCache

[evtimmy]

Right now ParseKeyValueCache has two configuration variables, but those have package visibility and can't be changed from the app.

The current config variables default to 2MB and 1000 requests:

    /* package */ static int maxKeyValueCacheBytes = DEFAULT_MAX_KEY_VALUE_CACHE_BYTES;
    /* package */ static int maxKeyValueCacheFiles = DEFAULT_MAX_KEY_VALUE_CACHE_FILES;

Simplest way is to expose these as public, but perhaps a better configuration API could be devised? Something along the lines of:

public static configure(maxSizeInBites, maxQueryCount)