Various fixes in setting globals in a script group

Bug: 25602504

1) Passing floating point values into a script group was broken,
since they were casted to long values. Fixed that in the frameworks
implementation by taking the raw bits instead.

2) Passing 64-bit values into a script group was broken on 32-bit
platforms, since they were casted to pointer-sized integers
(uintptr_t) in the JNI code. Fixed that by casting to int64_t
instead.

3) Setting global variables of Allocation type in a script group was
broken. The special size value -1 was used to indicate the value is an
Allocation. However, size was casted to size_t in the JNI code.
Fixed that by using signed integers.

Change-Id: Ifff099a76be7707df7b67c388395f5a00f9cae66