Introduce @TestApi BUGREPORT_MODE_MAX_VALUE

This cl introduces test API BUGREPORT_MODE_MAX_VALUE to make
bugreport mode validation more stable.

CtsCarrierApiTestCases has a test case to validate the supported
bugreport mode in AOSP. Without a reference to the range of supported
bugreport modes, the  test case can only use a hard-coded value which
is prone of failure when new mode is introduced.

Bug: 297892537
Test: atest BugreportManagerTest
Change-Id: I9369367ee4fcfb274f14c96bf501e9cd3eb98cbf