[frameworks][base][statsd] fix -Wimplicit-int-float-conversion

RAND_MAX cannot be precisely represented in an IEEE 754 single precision
float.  For the purpose of generating a random number, the imprecision
is irrelevant. Fix the warning with an explicit cast.

Bug: 139945549
Test: mm
Change-Id: Ica60b410bbbca0b3807f985e302abb0775133877
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>