Allow users to find all classes

QAndroidJniEnvironment::findClass is a convenient method needed to find
classes in all class loaders.

[ChangeLog][QtAndroid]
QAndroidJniEnvironment::findClass convenient method to find classes in
all class loaders.

Change-Id: I23c9e8a02aa7ca02cf707be5548ce1f8bb555dd7
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@qt.io>