Take as Unrestricted Mode in the UI if current Mode is Unknown.

- Battery usage mode MODE_UNKNOWN is (1) apps in allowlist and (2) app's OP_RUN_ANY_IN_BACKGROUND = MODE_IGNORED.
For this case, mode should be displayed as Unrestricted on UI since it is in allowlist; and should be logged as unknown mode for the potential compatibility issues.

- After Backup & Restore, the MODE_UNKNOWN will be reset to Optimized(Default) Mode on target device.

Bug: 372831500
Test: manual
Test: atest BatteryOptimizeUtilsTest
Test: atest AppOptModeSharedPreferencesUtilsTest
Flag: EXEMPT for simple fix
Change-Id: Ic18e026f71a4a50ffa1711e2ca0782aeb8b33076