Return incremented rescue level when bootlooping

The rescue level returned from onBootLoop must be incremented, since
otherwise USER_IMPACT_NONE would be returned to Package Watchdog if
the initial rescue level was 0, and no mitigation would take place.
Add a test to verify this.

Test: atest RescuePartyTest
Bug: 136135457
Change-Id: I27c56d17a6177abdeb3707fda70ca38d4fd4ef2c