ANDROID: revert some RNG function signature changes

v4.19.249 changed the len argument of get_random_bytes() from int to
size_t, and the buf argument of add_hwgenerator_randomness() from
'const char *' to 'const void *'.  As these functions are part of the
GKI ABI that must be preserved, restore the old signatures.
get_random_bytes() should never be called with a length greater than
INT_MAX anyway, so it's fine to keep the old signature.

Bug: 161946584
Fixes: 08b70638 ("random: use hash function for crng_slow_load()")
Change-Id: I0749bf3596eae9732e1ae2a8264ad0ca542dd3a3
Signed-off-by: Eric Biggers <ebiggers@google.com>