Fix getWifiManager exception if phone is encrypted

[Issue Details]
NetworkPolicyManagerService may invoke
dispatchRestrictBackgroundChanged after reboot if settings changed, and
invokes untetherAll in turn, which triggers ServiceNotFoundException
indirectly if the device is under encryption. The cause is that
WifiService is not running if the device is still locked.

[Solution]
In order to avoid ServiceNotFoundException in setWifiTethering, we add
check on return value of getWifiManager. Moreover, check of UsbManager
is also added to prevent potential similar issues of USB tethering.

Bug: 123504004
Test: FrameworksNetTests
  adb shell am instrument -w com.android.frameworks.tests.net
Test: CTS - CtsNetTestCases
  ANDROID_BUILD_droid-cts/tools/cts-tradefed run cts -m CtsNetTestCases
Test: CTS - CtsNetTestCasesLegacyApi22
  ANDROID_BUILD_droid-cts/tools/cts-tradefed run cts -m
  CtsNetTestCasesLegacyApi22
Test: CTS - CtsNetTestCasesLegacyPermission22
  ANDROID_BUILD_droid-cts/tools/cts-tradefed run cts -m
  CtsNetTestCasesLegacyPermission22

Change-Id: I2643188b51900f97ba58c8614c66cb55ebfaa2f6