FuelGauge waking up enhancement

[XuGuohui]

By applying the changes, device can resume from sleep faster if system power manager is enabled.

Example App

#include "Particle.h"

SYSTEM_MODE(MANUAL);

Serial1LogHandler log(115200, LOG_LEVEL_ALL);

void startup() {
    SystemPowerConfiguration conf;
    // conf.feature(SystemPowerFeature::DISABLE);
    System.setPowerConfiguration(conf);
}

STARTUP( startup() );

void setup() {
    System.sleep(SystemSleepConfiguration().mode(SystemSleepMode::STOP).duration(3s));
    LOG(TRACE, "Now: %d", millis());
}

void loop() {
}

Log if system power manager is not enabled:

0000004027 [app] application.cpp:17, setup(): TRACE: Now: 4027

Log if system power manager is enabled:

0000003537 [app] application.cpp:17, setup(): TRACE: Now: 3537

---

Completeness

• User is totes amazing for contributing!
• Contributor has signed CLA (Info here)
• Problem and Solution clearly stated
• Run unit/integration/application tests on device
• Added documentation
• Added to CHANGELOG.md after merging (add links to docs and issues)

[avtolstoy]

This should be done before or combined with update() call. As otherwise the fuelgauge will anyway be accessed in handleUpdate().