Fix sched_param initialization for musl libc

Musl libc defines struct sched_param with multiple fields, use an
empty initializer to initialize them all to default values instead
of trying to zero initialize a single field.  Fixes a build error
with USE_HOST_MUSL=true:
frameworks/native/services/sensorservice/aidl/SensorManager.cpp:204:38: error: missing field '__reserved1' initializer [-Werror,-Wmissing-field-initializers]
struct sched_param p = {0};

Test: m USE_HOST_MUSL=true libsensorserviceaidl
Change-Id: Ie36c59f802f393b1e71f058d7d2a00c64eb6b09e