Improved logging on HsumBootUserInitializer.

Changes:
 - Make DEBUG static as the class is small enough to justify the
   cost/benefit
 - Guarded all Slogf.d() calls with DEBUG
 - Changed a Slogf.d() call to Slogf.i()
 - Moved constructor logs to init()
 - Log value of relevant flags

Flag: EXEMPT logging changes
Bug: 417943524
Test: adb shell stop; adb shell setprop log.tag.HsumBootUserInitializer DEBUG;adb shell start;adb logcat HsumBootUserInitializer *:s

Change-Id: I03ddabe6e9c1a14a82a5f70fe7a64b78ab38d8b9