Added main layout for dive download

Similar to the subsurface downloadfromdivecomputer ui, main activity is made.
All the options required are added.

Signed-off-by: Venkatesh Shukla <venkatesh.shukla.eee11@iitbhu.ac.in>
View initialisation and scratching JNI

We are starting to scratch the surface of JNI. In this commit, we have taken all
the required views and initialised them. Also, a class contaning all native
functions is included. Right now, the aim it to populate the product and vendor
spinners by getting a list from libdivecomputer library.

Signed-off-by: Venkatesh Shukla <venkatesh.shukla.eee11@iitbhu.ac.in>
Building native parts for JNI

For making a function that gives back the String array, we are trying to use the
libdivecomputer library. These functions are meant to fill up the product and
vendor spinners on the application. NativeHelper is incidently a module in
bionic library and was cause of no JNI_OnLoad method errors. Hence it is removed
and the functions migrated to Main.

Signed-off-by: Venkatesh Shukla <venkatesh.shukla.eee11@iitbhu.ac.in>
Win32 compilation of JNI functions

JNIEXPORT and JNICALL are necessary for compilation on win32 systems that
require special keyword for functions exported from dynamic link libraries.

Signed-off-by: Venkatesh Shukla <venkatesh.shukla.eee11@iitbhu.ac.in>