Android: Store and use the class names as key when caching.

Previously the jclass handle was part of the key used for caching the
class' methods and fields. Using the jclass handle is not ideal, but
it meant that we could easily create a key when the only identifier we
had was the jobject or jclass handle. However, in Android 5.1, the
re-use of handles seems to be more aggressive and therefore increasing
the chance of a collision in the cache look-up.

This change removes caching for all calls where we don't know the class
name, as that is the only thing that guarantees that we create unique
keys for each class. The consequence of this is that only calls that
provide a class name will benefit from the internal caching.

Task-number: QTBUG-45748
Change-Id: I0039d04e7c068debc9e3b3983632c45dc8e52309
Reviewed-by: Frank Meerkoetter <frank.meerkoetter@basyskom.com>
Reviewed-by: Alex Blasche <alexander.blasche@theqtcompany.com>