entity-manager: create class PowerStatusMonitor

Removing following global variables
```
// NOLINTNEXTLINE(cppcoreguidelines-avoid-non-const-global-variables)
static bool powerStatusOn = false;
// NOLINTNEXTLINE(cppcoreguidelines-avoid-non-const-global-variables)
static std::unique_ptr<sdbusplus::bus::match_t> powerMatch = nullptr;
```
and placing them in a new class PowerStatusMonitor.

class PowerStatusMonitor is a member of class EntityManager and
inherits its lifetime. So there should be no issue with the lifetime
compared to the global variable.

Tested: Inspection only.

Change-Id: Ibdd9eb3adb6655d88c90c2da8435912887ff547c
Signed-off-by: Alexander Hansen <alexander.hansen@9elements.com>
diff --git a/src/entity_manager/entity_manager.cpp b/src/entity_manager/entity_manager.cpp
index ff9a5d5..d55105c 100644
--- a/src/entity_manager/entity_manager.cpp
+++ b/src/entity_manager/entity_manager.cpp
@@ -385,7 +385,7 @@
         return;
     }
 
-    if (!em_utils::isPowerOn() && scannedPowerOff)
+    if (!powerStatus.isPowerOn() && scannedPowerOff)
     {
         return;
     }
@@ -398,7 +398,7 @@
                 return;
             }
 
-            bool powerOff = !em_utils::isPowerOn();
+            bool powerOff = !powerStatus.isPowerOn();
             for (const auto& [name, device] : lastJson.items())
             {
                 pruneDevice(systemConfiguration, powerOff, scannedPowerOff,
@@ -505,7 +505,7 @@
                 [this, count, oldConfiguration, missingConfigurations]() {
                     // this is something that since ac has been applied to the
                     // bmc we saw, and we no longer see it
-                    bool powerOff = !em_utils::isPowerOn();
+                    bool powerOff = !powerStatus.isPowerOn();
                     for (const auto& [name, device] :
                          missingConfigurations->items())
                     {
@@ -693,7 +693,7 @@
 
     // some boards only show up after power is on, we want to not say they are
     // removed until the same state happens
-    em_utils::setupPowerMatch(em.systemBus);
+    em.powerStatus.setupPowerMatch(em.systemBus);
 
     io.run();