[Table logging] Add "**" to logs that were generated when a flow

was initially collected.

Fixes: 272518407
Test: `adb shell dumpsys activity service
com.android.systemui/.SystemUIService WifiTableLog` -> see ** on the
starting values, then don't see ** on the rest of the values
Test: same for `MobileSummaryLog`
Test: same for `MobileConnectionLog[5]` (5 = my personal sub ID)
Test: atest LogDiffsForTableTest TableChangeTest TableLogBufferTest

Change-Id: I6d23f02c13dad77f4b2aa63fff2f37105512f79d