PRESUBMIT.py: Revisit default handling of BanRule::treat_as_error

The docs state [1] the default value of None is equivalent to True.
BanRules are used in 2 checks:
* _CheckAndroidNoBannedImports(): Here the docs *happen* to be correct,
  because all current rules leave the field unspecified [2] and the
  function issues errors unconditionally without ever reading the
  field [3].
* CheckNoBannedFunctions(): Here the docs are incorrect, None is treated
  like False [4].

Given that, change the following:
* Get rid of the None nonsense, we can just specify a default boolean
  value directly. Do this in such a way that, after this CL, warnings
  remain warnings and errors remain errors.
* Merge _CheckAndroidNoBannedImports() into CheckNoBannedFunctions(),
  modulo renaming the function and tweaking output strings. The
  duplication was the underlying root cause for the discrepancy and
  there was already a TODO about merging them.
  One consequence is the imports are checked on both commit and upload
  now (before the CL, _CheckAndroidNoBannedImports() was called by
  ChecksAndroidSpecificOnUpload(), which only triggered on upload).
  Another consequence is that now it's possible to add banned imports
  that issue presubmit warnings and not errors.

[1] https://source.chromium.org/chromium/chromium/src/+/main:PRESUBMIT.py;l=130;drc=9100968543997df5923fa4918179b6be1b7c6a3a
[2] https://source.chromium.org/chromium/chromium/src/+/main:PRESUBMIT.py;l=140-170;drc=9100968543997df5923fa4918179b6be1b7c6a3a
[3] https://source.chromium.org/chromium/chromium/src/+/main:PRESUBMIT.py;l=3033;drc=9100968543997df5923fa4918179b6be1b7c6a3a
[4] https://source.chromium.org/chromium/chromium/src/+/main:PRESUBMIT.py;l=2937;drc=9100968543997df5923fa4918179b6be1b7c6a3a

Change-Id: I1cba6be74b5ab11f5924660073664fd4c123979b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7006085
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Commit-Queue: Andrew Grieve <agrieve@chromium.org>
Auto-Submit: Victor Vianna <victorvianna@google.com>
Cr-Commit-Position: refs/heads/main@{#1524551}