Use distinct value for BAL_BLOCK

Instead of logging 0 which can be misinterpreted as "missing data" in
protos, use a distinct value defined in the proto enum.

Test: atest BackgroundActivityStartController*Test
Flag: com.android.window.flags.bal_improved_metrics
Bug: 339245692
Change-Id: I6126990f9668e35fe876ceae3cdf94e8f407bcad
1 file changed